Patent Publication Number: US-2009237877-A1

Title: Storage subsystem

Description:
CROSS-REFERENCE TO PRIOR APPLICATION 
     This application is a Continuation of U.S. patent application Ser. No. 12/320,597 filed on Jan. 29, 2009, which is a Reissue application Ser. No. 10/967,361 which issued into U.S. Pat. No. 7,359,186 issued on Apr. 15, 2008. Priority is claimed from U.S. patent application Ser. No. 12/320,597 filed on Jan. 29, 2009, which claims priority to U.S. Pat. No. 7,359,186 issued on Apr. 15, 2008, which claims the priority of Japanese Patent Application No. 2004-251940 filed on Aug. 31, 2004, all of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a storage subsystem that comprises a plurality of storage devices. 
     BACKGROUND OF THE INVENTION 
     As a storage subsystem that comprises a plurality of storage devices, the storage subsystem disclosed by Japanese Patent Application Laid Open No. 2004-022058, for example, is known. This storage subsystem comprises a deep enclosure. Storage devices, which are inserted in the depth direction via the front side, are arranged in the enclosure. In other words, the user is able to install a storage device in the storage subsystem by inserting a storage device in the enclosure in the depth direction via the front side. 
     SUMMARY OF THE INVENTION 
     There is a desire for miniaturization of storage subsystems. Further, small-form factor hard disk drives (so-called 2.5-inch HDD developed for enterprises, for example) have been produced. A method that implements miniaturization of a storage subsystem by adopting such small-form factor hard disk drives (abbreviated to ‘SFF-HDD’ hereinafter) as the storage devices of the storage subsystem has been considered. However, simply changing the installed storage device to an SFF-HDD while retaining the conventional storage-device installation method is considered inadequate from at least one perspective among a variety of perspectives such as high-density mounting of the storage device, more efficient cooling of the storage subsystem and maintenance of the storage subsystem, for example. 
     Therefore, an object of the present invention is to provide a method of mounting storage devices that serves at least one of the miniaturization of the storage subsystem, an increased capacity, and a higher performance. 
     Another object of the present invention is to mount a plurality of storage devices in a storage subsystem of a high density. 
     A further object of the present invention is to raise the cooling efficiency of the storage subsystem. 
     Another object of the present invention is to simplify maintenance-related manipulation of the storage subsystem and to increase stability or reliability. 
     Further objects of the present invention will become evident from the following description. 
     A storage subsystem according to a first aspect of the present invention is connected to an external device (Host Computer, for example) and comprises a storage device arrangement portion on which a plurality of storage devices is arranged; and a control device that controls communications between the plurality of storage devices arranged on the storage device arrangement portion and the external device, wherein the storage device arrangement portion is constituted such that the plurality of storage devices is arranged upright in the directions of two dimensions. For example, the storage device arrangement portion comprises a substrate for the arrangement of storage devices and has a mounting portion (Connector, for example) for mounting the plurality of storage devices on the substrate. 
     Further, ‘upright’ storage devices means, for example, that the depth direction of the storage devices runs in the vertical direction. In other words, when the storage devices have a first face for which the depth direction is the normal direction and a second face, for example, a state where the first face is oriented vertically upward and the second face is oriented vertically downward is the upright state of the storage device. Further, in this case, a connector for connecting the storage device to a substrate or the like may be provided on the first face or the second face. 
     In a first embodiment, the storage subsystem further comprises a enclosure for housing the storage device arrangement portion; and an arrangement portion displacement mechanism that displaces the storage device arrangement portion inside and outside the enclosure. The arrangement portion displacement mechanism can displace the storage device arrangement portion in the directions of two dimensions (the depth direction of the enclosure and the reverse direction or the transverse direction of the enclosure, for example) for example. 
     In a second embodiment, the storage subsystem according to the first embodiment is such that the storage device arrangement portion comprises a first sub-arrangement portion for arranging two or more first storage devices among the plurality of storage devices; and a second sub-arrangement portion for arranging two or more second storage devices among the plurality of storage devices. The arrangement portion displacement mechanism displaces the first sub-arrangement portion and the second sub-arrangement portion separately. 
     In a third embodiment, the storage device arrangement portion comprises a plurality of storage device slots corresponding with a plurality of storage devices respectively and is constituted to arrange a plurality of storage devices, each of which is inserted via the plurality of storage device slots. Each of the plurality of storage device slots is constituted to receive a storage device in an upright state vertically from above. 
     In a fourth embodiment, the storage subsystem further comprises a cooling portion that causes a gas for cooling the storage devices arranged on the storage device arrangement portion to flow to the storage device arrangement portion. The storage device arrangement portion is constituted such that a plurality of storage device columns consisting of two or more storage devices that follow the direction in which the gas flows are formed and the plurality of storage device columns are at equal intervals. 
     In a fifth embodiment, the plurality of storage devices includes a low-heat storage device that emits heat by consuming first electrical power and a high-heat storage device that emits heat that is of a higher temperature than the heat of the low-heat storage device by consuming second electrical power. The storage subsystem further comprises a cooling portion that causes a gas for cooling the storage devices arranged on the storage device arrangement portion to flow to the storage device arrangement portion. The storage device arrangement portion is constituted such that the low-heat storage device is disposed upstream in the direction in which the gas flows and the high-heat storage device is disposed downstream in the direction in which the gas flows. Further, the first and second electrical power may be the same or different. 
     In a sixth embodiment, the storage subsystem further comprises a cooling portion that causes a gas for cooling the storage devices arranged on the storage device arrangement portion to flow to the storage device arrangement portion; and a storage device dummy that is disposed on the storage device arrangement portion so that the flow of gas is not disturbed when the maximum number of storage devices that can be arranged is not arranged on the storage device arrangement portion. 
     In a seventh embodiment, the storage subsystem further comprises a cooling portion that causes a gas for cooling the storage devices arranged on the storage device arrangement portion to flow to the storage device arrangement portion. The storage device arrangement portion is constituted such that a plurality of storage device columns consisting of two or more storage devices that follow the direction in which the gas flows are formed and the width of at least one storage device column among the plurality of storage device columns is narrower downstream than upstream in the direction in which the gas flows. 
     In an eighth embodiment, the storage device arrangement portion is constituted such that a plurality of storage device columns consisting of two or more storage devices are formed. The storage subsystem further comprises a plurality of operating portions corresponding with the plurality of storage device columns respectively, wherein the operation of an operating portion that is selected by a user from among the plurality of operating portions is detected and the user is allowed to remove a storage device that belongs to the storage device column corresponding with the selected operating portion. 
     In a ninth embodiment, the storage subsystem according to the eighth embodiment further comprises a enclosure for housing the storage device arrangement portion; and an arrangement portion displacement mechanism that displaces the storage device arrangement portion inside and outside the enclosure and in the directions of the two dimensions. The storage device column is a column formed in the same direction as the displacement direction of the storage device arrangement portion. 
     In a tenth embodiment, the storage device arrangement portion comprises a plurality of arrangement positions corresponding with the plurality of storage devices respectively. The control device comprises a storage region that stores control information indicating where in the plurality of arrangement positions which types of storage devices are arranged and indicating the respective states of each of the storage devices; and a control portion that displays a GUI screen. The control portion prepares a plurality of display positions on the GUI screen corresponding with the plurality of arrangement positions respectively, displays a graphic representing an arranged storage device in each of the plurality of display positions, and displays at least one of the type and state of the storage device corresponding with the graphic on the GUI screen so that the type and/or state is associated with the graphic. 
     The storage subsystem according to a second aspect of the present invention is a storage subsystem that is connected to an external device and that possesses depth, comprising a storage device arrangement portion on which a plurality of storage devices is arranged upright in the directions of two dimensions that include the depth direction of the storage subsystem; a control device that controls communications between the plurality of storage devices arranged on the storage device arrangement portion and the external device; a cooling portion that causes a gas for cooling the storage devices arranged on the storage device arrangement portion to flow to the storage device arrangement portion; a enclosure for housing the storage device arrangement portion, the control device and the cooling portion; and an arrangement portion displacement mechanism that displaces the storage device arrangement portion inside and outside the enclosure and in the directions of the two dimensions. The storage device arrangement portion is constituted such that a plurality of storage device columns consisting of two or more storage devices that follow the direction in which the gas flows are formed so that the plurality of storage device columns are at equal intervals or so that the width of at least one storage device column among the plurality of storage device columns is narrower downstream than upstream in the direction in which the gas flows. The storage subsystem further comprises a plurality of operating portions corresponding with the plurality of storage device columns respectively, wherein the operation of an operating portion that is selected by a user from among the plurality of operating portions is detected and the user is allowed to remove a storage device that belongs to the storage device column corresponding with the selected operating portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  provides a schematic external view of a storage subsystem according to a first embodiment example of an embodiment of the present invention; 
         FIG. 2A  shows a three-dimensional outline of the internal constitution of the basic enclosure  1 A; 
         FIG. 2B  is a block diagram that represents the internal constitution of the basic enclosure  1 A when the basic enclosure  1 A is viewed from above; 
         FIG. 3  provides a schematic view of the constitution of the cross-section  3 - 3  of the basic enclosure  1 A in  FIG. 2B ; 
         FIG. 4A  provides an enlarged view of the HDD slot  83  in  FIG. 2B  and of the vicinity thereof; 
         FIG. 4B  provides a schematic of a cross section of an HDD  23  that has been inserted completely via the HDD slot  83 ; 
         FIG. 5A  schematically shows an external view in a case where the HDD group installation drawer  19  is withdrawn to a certain extent from the basic enclosure  1 A; 
         FIG. 5B  schematically shows the cross section  5 B- 5 B in  FIG. 5A ; 
         FIG. 6A  shows the appearance of the flow of an air stream when an HDD  23  is mounted in all the HDD mounting portions  31  of the HDD group installation drawer  19 ; 
         FIG. 6B  is an enlarged view of an intercolumn path in  FIG. 6A ; 
         FIG. 7A  shows an example of one variation on the cooling design; 
         FIG. 7B  shows an example of another variation on the cooling design; 
         FIG. 8  shows an example of the constitution within the controller units  37 A and  37 B, an example of the constitution within the switch enclosure  3 , an example of the constitution within the expansion enclosure  1 B, and an example of the constitution of the connection between the controller units  37 A,  37 B and the expansion enclosure  1 B; 
         FIG. 9  shows an example of the constitution of the network in a case where the storage subsystem  1  is connected to a communication network; 
         FIG. 10A  shows an example of a GUI screen that is displayed on a management terminal  203  when a fault has not occurred; 
         FIG. 10B  shows an example of a GUI screen that is displayed on the management terminal  203  when a fault has occurred; 
         FIG. 11  shows an example of the flow of processing that is executed up until a fault is recovered after a fault occurs with an HDD; 
         FIG. 12A  shows a first variation on the integrated withdrawal method; 
         FIG. 12B  shows a second variation on the integrated withdrawal method; 
         FIG. 12C  shows a third variation on the integrated withdrawal method; 
         FIG. 13A  provides an outline of one variation on the separate withdrawal method; 
         FIG. 13B  serves to illustrate the variation in detail; 
         FIG. 14A  shows a first variation on the removal method for the HDD  23 ; 
         FIG. 14B  shows a second variation on the removal method for the HDD  23 ; 
         FIG. 15A  shows a first variation on the cooling design for increasing the velocity of the cooling air stream on the rear side; 
         FIG. 15B  shows a second variation on the cooling design for increasing the velocity of the cooling air stream on the rear side; 
         FIG. 16  shows one variation on the constitution within the expansion enclosure  1 B; 
         FIG. 17  shows another variation on the constitution in the expansion enclosure  1 B; 
         FIG. 18  shows yet another variation on the constitution within the expansion enclosure  1 B and one variation on the constitution of the connection between the controller units  37 A,  37 B and each HDD  23 ; 
         FIG. 19  shows the constitution of the HDD group installation drawer in the expansion enclosure  1 B in detail; and 
         FIG. 20  shows an example in which the columns constituted by the SATA-HDD  23 A and the columns constituted by the SAS-HDD  23 S are arranged alternately. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A storage subsystem relating to an embodiment of the present invention will be described hereinbelow with reference to the drawings. A variety of storage devices, such as hard disk drives (abbreviated to ‘HDD’ hereinafter), DVD (Digital Versatile Disks) drives or magnetic tape drives, for example, can be adopted as the storage devices that are installed in the storage subsystem. The storage devices are referred to as ‘HDD’ hereinbelow. 
     First Embodiment Example 
       FIG. 1  provides a schematic external view of a storage subsystem according to a first embodiment example of an embodiment of the present invention. 
     A storage subsystem  1  is a RAID (Redundant Array of Inexpensive Disks) system, for example. The storage subsystem  1  comprises a basic enclosure  1 A, a plurality of (or a single) expansion enclosures  1 B,  1 B, . . . , and a switch enclosure (abbreviated to ‘SW enclosure’ hereinafter)  3  that electrically connects the basic enclosure  1 A and expansion enclosures  1 B. 
     The SW enclosure  3  comprises a switch device (not shown). An HDD connection portion (not shown) within the expansion enclosure  1 B, which will be described subsequently, is connected to the switch device via a cable  15 . Further, controller units (not shown) in the basic enclosure  1 A (described subsequently) are connected to the switch device via cables  13 . As a result, the controller units are able to access the HDDs in the expansion enclosure  1 B via the switch device and the HDD connection portion within the expansion enclosure  1 B. 
     The expansion enclosure  1 B comprises, on the front side thereof, an HDD group housing space  9  for housing an HDD group comprising a plurality of HDD and comprises, on the rear side thereof, an HDD connection portion housing space  11  for housing an HDD connection portion. The HDD connection portion is connected to the switch device within the SW enclosure  3  via the cable  15 . 
     The basic enclosure  1 A comprises, on the front side thereof, an HDD group housing space  5  for housing an HDD group and comprises, on the rear side thereof, a controller housing space  7  for housing a controller unit, fan unit (not shown), and so forth of the storage subsystem  1 . The controller unit is connected electrically to the HDD in the HDD group housing space  5  and is connected electrically to the switch device in the SW enclosure  3  via a cable  13 . Accordingly, the controller unit is able to access the HDD in the HDD group housing space  5  and is able to access the HDD in the expansion enclosure  1 B via the switch device in the SW enclosure  3  and the HDD connection portion within the expansion enclosure  1 B. 
     Further, the SW enclosure  3  may be left out of the storage subsystem  1 . In this case, the controller unit in the basic enclosure  1 A may be connected to the HDD connection portion in the expansion enclosure  1 B via a cable. Further, an HDD  23  may be of any size (a 3.5-inch HDD or 2.5-inch HDD is acceptable, for example). 
     Further, in the storage subsystem  1 , the switch device (not shown) in the SW enclosure  3  may be installed in a enclosure  3  separate from the basic enclosure  1 A or expansion enclosure  1 B, as mentioned earlier, or may be installed in the basic enclosure  1 A or expansion enclosure  1 B. In the former case, the expansion capability is high and, in the latter case, miniaturization of the storage subsystem  1  is feasible because same is complete even if the SW enclosure  3  is not provided. 
       FIG. 2A  shows a three-dimensional outline of the internal constitution of the basic enclosure  1 A.  FIG. 2B  is a block diagram that represents the internal constitution of the basic enclosure  1 A when the basic enclosure  1 A is viewed from above.  FIG. 3  provides a schematic view of the constitution of the cross-section  3 - 3  of the basic enclosure  1 A in  FIG. 2B . 
     The basic enclosure  1 A is a rectangular parallelepiped with an internal space and has depth from the front side toward the rear side. The internal space of the basic enclosure  1 A is divided into the HDD group housing space  5  and the controller housing space  7 . A back plane  17  is provided between the HDD group housing space  5  and the controller housing space  7  and the HDD group housing space  5  and controller housing space  7  are partitioned by the back plane  17 . 
     An HDD group installation drawer  19  for installing an HDD group is provided within the HDD group housing space  5 . A drawer slide mechanism  27  that enables the HDD group installation drawer  19  to slide in the depth direction and in the reverse direction (that is, in both directions toward the rear side and front side) is provided in the basic enclosure  1 A. The drawer slide mechanism  27  may be any mechanism as long as same allows the HDD group installation drawer  19  to slide. As one example, the drawer slide mechanism  27  comprises a rail  25  on which the HDD group installation drawer  19  is placed, and a plurality of guide rollers  29  that guide the rail  25  in both directions toward the front and rear sides of the basic enclosure  1 A, as shown in  FIG. 3 . The plurality of guide rollers  29  stand in a line in the depth direction. The provision of such a drawer slide mechanism  27  makes it possible to suppress vibrations of the HDD  23  and thus prevent problems such as system stoppage from occurring. Further, a stopper for limiting the distance over which the HDD group installation drawer  19  is withdrawn may be provided in the basic enclosure  1 A. The front face of the basic enclosure  1 A may be a door that can be opened and closed that may be open beforehand. As a result, the HDD group installation drawer  19  can be inserted and withdrawn. 
     The HDD group installation drawer  19  is a rectangular-parallelepiped-shaped box with depth, for example (any shape is permissible as long as the shape has depth). A plurality of HDD  23  is arranged in the HDD group installation drawer  19  in the directions of two dimensions. More specifically, two or more HDDs  23  are arranged from the front side toward the rear side (that is, in the depth direction) in the HDD group installation drawer  19 , for example. Here, the depth direction of the HDD  23  is orthogonal to the depth direction of the basic enclosure  1 A and is parallel to the height direction of the basic enclosure  1 A. To state this another way, two or more HDDs  23  are arranged in a standing state in the depth direction of the basic enclosure  1 A within the HDD group installation drawer  19 . In other words, each HDD  23  is a rectangular parallelepiped the vertical dimension of which is the longest, the horizontal dimension of which is the second-longest, and the height of which is the third-longest. The respective HDDs  23  are arranged such that the vertical of the HDD  23  is parallel to the height of the basic enclosure  1 A and the horizontal of the HDD  23  is parallel to the depth of the basic enclosure  1 A. Accordingly, the HDD  23  can be provided at a high density within a fixed space. According to the present embodiment, two or more (four, for example) HDDs  23  are arranged in the depth direction in the HDD group installation drawer  19  and two or more (twenty-two, for example) HDDs  23  are arranged in the width direction, for example. Further, in this embodiment, the HDDs  23  may be arranged such that the width of the HDDs  23  follows the depth direction of the basic enclosure  1 A, but may instead be arranged such that the height of the HDD  23  follows the depth direction of the basic enclosure  1 A. Hereinbelow, an arrangement of two or more HDDs  23  in the depth direction of the basic enclosure  1 A (that is, in the vertical width direction of the basic enclosure  1 A is sometimes referred to as a ‘column’ or ‘HDD column’ and an arrangement of two or more HDDs  23  in the transverse width direction) is sometimes referred to as a ‘row’ or ‘HDD row’. An HDD column may be parallel to the depth direction or inclined with respect to the depth direction. Likewise, an HDD row may be parallel to the transverse width direction or inclined with respect to the depth direction. Furthermore, the distance between an HDD column and an adjacent HDD column may be the same between all the columns or may be different. Likewise, the distance between an HDD row and an adjacent HDD row may be the same between all the rows or may be different. 
     An HDD group mounting substrate  21  is laid on the bottom of the HDD group installation drawer  19 . The HDD group mounting substrate  21  is a printed substrate on which a wiring pattern has been printed, for example. A plurality of HDD mounting portions  31  (connectors that are embedded in the substrate  21 , for example), to which connectors  32  of the HDD  23  (or connectors of canisters in which the HDD  23  are installed) are physically connected, are provided densely over the whole area of the HDD group mounting substrate  21 . Stated using the example in  FIGS. 2A ,  2 B, and  3 , two or more (four, for example) HDD mounting portions  31  are arranged in the row direction (width direction) on the HDD group mounting substrate  21  and two or more (twenty-two, for example) HDD mounting portions  31  are arranged in the column direction (depth direction). Further, a plurality of HDDs  23  mounted in the HDD group installation drawer  19  may be constituted by only HDDs of one type (HDDs of the same standard, for example), or may be constituted by HDDs of a plurality of types. In this first embodiment example, two types are mixed, namely SATA (Serial ATA)-standard HDDs (‘SATA-HDDs’ hereinafter) and SAS (Serial Attached SCSI)—standard HDDs (‘SAS-HDDs’ hereinafter). 
     As shown in  FIGS. 2B and 3 , a plurality of HDD slots  83  arranged in the directions of two dimensions (in the row and column directions, for example) is provided on the upper face of the HDD group installation drawer  19 . HDDs  23  are inserted in the HDD slots  83 . The HDD  23  has a front face and rear face, the depth direction of the HDD  23  being the normal direction, and has a connector  32  on the rear face, for example. When the HDD  23  is completely inserted in the HDD  83  with the rear face of the HDD  23  oriented vertically downward, the connector  32 , which is provided on the rear face of the HDD  23 , is physically connected to the HDD mounting portion  31  in the HDD group mounting substrate  21 . Further, a door, which is in a closed state when the HDD  23  is pressed into the opening of the HDD slot  83  to open same and there is no HDD  23  and/or when the HDD  23  is inserted completely, for example, may be provided. The provision of such a door makes it possible to prevent dust and so forth from entering via the HDD slot  83 . 
     One or more (two, for example) first state display lamps (LED, for example)  81 A and  81 B and an eject button  85  are provided close to each HDD slot  83  (near the front side of the opening of the HDD slot  83 , for example) in the upper face of the HDD group installation drawer  19 .  FIG. 4A  provides an enlarged view of the HDD slot  83  in  FIG. 2B  and of the vicinity thereof, while  FIG. 4B  provides a schematic of a cross section of an HDD  23  that has been inserted completely via the HDD slot  83 . The left-hand figure in  FIG. 4B  shows an eject mechanism  86  and HDD  23  as viewed from the side of the basic enclosure  1 A and the right-hand figure in  FIG. 4B  shows the eject mechanism  86  and HDD  23  as viewed from the front face of the basic enclosure  1 A. Controller units  37 A and  37 B, which will be described subsequently, are able to ignite and turn off at least one of the first-state display lamps  81 A and  81 B that correspond with the HDD slot  83  comprising the HDD  23  in accordance with the state of a certain HDD  23  (access in progress, for example), for example. When the eject button  85  is operated, the HDD  23  in the HDD slot  83  corresponding with the eject button  85  is extracted via the HDD slot  83  by means of the eject mechanism  86 . Further, the extraction of the HDD  23  may be performed by means of computer processing of the controller units  37 A and  37 B when a signal indicating that the eject button  85  has been pressed is sent to the controller units  37 A and  37 B, for example, or may be executed by means of a mechanical constitution not requiring computer processing. Further, removal of the HDD  23  is not limited to a method that adopts the eject mechanism  86 . Several variations may be considered. These variations will be described in the third embodiment example (described subsequently). 
     As shown in  FIG. 3 , a lock mechanism  87  is provided for each HDD slot  83 . The lock mechanism  87  is a mechanism that controls the insertion of the HDD  23  into the corresponding HDD slot  83  and/or the removal of the HDD  23  from the HDD slot  83 . When the lock mechanism  87  has locked the HDD slot  83 , removal of the HDD  23  via the HDD slot  83  is not possible and, when the lock is released, removal of the HDD  23  via the HDD slot  83  is permitted. A variety of methods can be adopted as the method of locking the HDD slot  83 . 
     The HDD group installation drawer  19  has a second state display lamp  33  and a lock control button  35  provided for each column as shown in  FIG. 2A . The second state display lamp (LED, for example)  33  and lock control button  35  are electrically connected to the controller unit  37 A (and/or  37 B) via the HDD group mounting substrate  21 , for example. In a case where the occurrence of an anomaly is detected with at least one of the two or more HDDs  23  belonging to a certain column, the controller unit  37 A ignites the second state display lamp  33  that corresponds with this column. Further, upon detecting that the lock control button  35  corresponding with a certain column has been operated, the controller unit  37 A enables the mounting and/or removal of the HDD  23  belonging to the column and prohibits the mounting and/or removal of the HDD  23  belonging to each of the other columns. To describe this more specifically, upon detecting that the lock control button  35  corresponding with a certain column has been operated, the controller unit  37 A controls each lock mechanism  87  corresponding with the HDD slots  83  belonging to the column to enable removal (or mounting) of the HDDs  23  belonging to the column and controls each lock mechanism  87  corresponding with the HDD slots  83  belonging to each of the other columns to prohibit removal (or mounting) of the HDDs  23  via the HDD slots  83  belonging to each of the other columns, for example. As a result, unless the lock control button  35  corresponding with the desired column is pressed, the HDDs  23  cannot be removed via the HDD slots  83  belonging to the column. Hence, the unintentional removal of HDDs  23  by mistake can be prevented. 
     A connector (referred to as a ‘back plane front-face connector’ hereinbelow)  47  that is connected to the front face of the back plane  17  is provided on the rear side of the HDD group installation drawer  19 . The back plane front-face connector  47  is electrically connected to the HDD group mounting substrate  21  (may be physically connected). 
     In the middle area of the controller housing space  7 , multiplexed (duplexed, for example) controller units  37 A and  37 B and a power supply unit  39 , which constitutes a secondary power supply for the controller units  37 A,  37 B and the HDDs  23  in the basic enclosure  1 A, is provided. The controller units  37 A and  37 B and the power supply unit  39  are arranged in a stacked configuration. For example, the power supply unit  39  is located in the upper layer and the controller units  37 A and  37 B are located in a layer below the battery  39 . A connector is provided on the front and rear sides of each of the controller units  37 A,  37 B and power supply unit  39 . A front-side connector  41  is physically connected (or electrically connected) to the rear side  17 B of the back plane  17  and a rear-side connector  43  is physically or electrically connected to a variety of targets (communication network, back plane  17 , the switch device in the SW enclosure  3  or mains power supply, for example). As a result, the power supply unit  39  can supply electrical power to each of the HDDs  23 , each of the controller units  37 A and  37 B, and/or each of fans  45 A,  45 B (described subsequently) via the back plane  17 . Further, one controller unit  37 A (or  37 B) is able to access the other controller unit  37 B (or  37 A) via the front-side connector  41  and back plane  17 . Furthermore, the controllers  37 A and  37 B are able to access an optional HDD  23  via the rear-side connector  43  (or front-side connector  41 ), the back plane  17  and the HDD group mounting substrate  21  and are able to access an HDD in the expansion enclosure  1 B via the SW enclosure  3 . The controller units  37 A and  37 B are also able to receive a read command or write command from a host device via the rear-side connector  43  and then access an optional HDD  23  via the back plane  17  in response to this read command or write command. The controller units  37 A and  37 B may also execute specified fault processing when a maintenance/replacement mode is executed (when it is detected that the lock control button corresponding with a certain column has been pressed, for example). A specific example will be provided below. For example, a vibration sensor  71 , which outputs a signal with a value corresponding to the vibration of the HDD group installation drawer  19  to the controller units  37 A and  37 B, is provided on the HDD group mounting substrate  21 . The controller units  37 A and  37 B are provided with a vibration threshold value storage area  73  (provided in the memory of the controller circuit (mentioned subsequently) in the controller unit  37 A, for example) for storing a threshold value for the signal value from the vibration sensor  71  (‘vibration threshold value’ hereinafter). The vibration threshold value is set at a value that is sufficiently higher than the signal value outputted in accordance with the vibration that occurs when the HDD group installation drawer  19  is withdrawn or inserted. The controller units  37 A and  37 B compare the value of the signal received from the vibration sensor  71  with the vibration threshold value that is stored in the vibration threshold value storage area  73 . When the value of the received signal is lower than the vibration threshold value, the controller units  37 A and  37 B judge that the HDD group installation drawer  19  has been withdrawn or pushed in and the specified fault processing corresponding with this judgment result is executed. When the value of the received signal is higher than the vibration threshold value, the controller units  37 A and  37 B judge that an error has occurred and normal fault processing is executed. Where normal fault processing is concerned, the controller units  37 A and  37 B execute processing to move data in each HDD  23  to an HDD  23  in another enclosure (the expansion enclosure  1 B, for example), for example. On the other hand, where specified fault processing is concerned, the controller units  37 A and  37 B access an optional HDD  23  in response to a write command or read command from a host device and, when this access fails, the controller units  37 A and  37 B execute a higher number of retries than the normal number (that is, the number of times a retry is executed is then greater than the normal number). 
     Fan units  45 A and  45 B are provided to the left and right of the middle area of the controller housing space  7 . The fan units  45 A and  45 B are duplexed fan units. The fan units  45 A and  45 B comprise a connector on both the front and rear sides. A front-side connector  51  is connected physically or electrically to the rear face  17 B of the back plane  17 . As a result, the fan units  45 A and  45 B can be driven as a result of receiving a supply of electrical power via a rear-side connector  53  and this driving allows air to be introduced from the front face of the basic enclosure  1 A via the HDD group installation drawer  19 , for example. As a result, the interior of the HDD group installation drawer  19  is cooled. 
     The back plane  17  is a printed substrate printed with a wiring pattern that has a front face  17 S and a rear face  17 B. A connector  48  to which the connector  47  of the HDD group installation drawer  19  is physically or electrically connected is provided on the front face  17 S of the back plane  17 . A connector  55 , to which the connectors  41  of the power supply unit  39  and controller units  37 A and  37 B are physically or electrically connected, and a connector  57 , to which the connectors  51  of the fan units  45 A and  45 B are physically or electrically connected, are provided on the rear face  17 B of the back plane  17 . The connectors  48 ,  55 , and  57 , which are provided on the back plane  17 , are embedded in the back plane  17 , for example. Because the HDD group installation drawer  19 , in which the HDDs  23  are mounted, the controller units  37 A and  37 B, and so forth, are connected to the back plane  17 , the controller units  37 A and  37 B are able to access an optional HDD  23  via the back plane  17  and HDD group mounting substrate  21 . Further, as will be described subsequently, the back plane  17  has a hole  61  for transmitting air, which has been introduced from the front face of the basic enclosure  1 A and that has passed within the HDD group installation drawer  19 , to the rear side. As shown in  FIG. 3 , the insertion and extraction direction of the HDD  23  in the HDD group installation drawer  19  is a direction that transects (is orthogonal to, for example) the direction of air flow (indicated by the dotted line). The HDD mounting portions  31  are arranged in the direction of air flow and not opposing the direction of air flow. As a result, it is possible to prevent dust from falling onto the HDD group mounting substrate  21  and hence the occurrence of contact problems between the HDD group mounting substrate  21  and HDD  23 , and so forth. 
     An outline of the constitution and functions of the basic enclosure  1 A was provided above. The constitution in the HDD group housing space  5  can also be applied to the HDD group housing space  9  of the expansion enclosure  1 B. 
     Furthermore, a description will be provided below for the installation and removal of the HDD  23  in and from the HDD group installation drawer  19  when the HDD group installation drawer  19  of the basic enclosure  1 A is withdrawn or pushed in. 
       FIG. 5A  schematically shows an external view in a case where the HDD group installation drawer  19  is withdrawn to a certain extent from the basic enclosure  1 A.  FIG. 5B  schematically shows the cross section  5 B- 5 B in  FIG. 5A . Further, the cross-sectional view shown in  FIG. 5B  is a more schematic view than that of  FIG. 3 . 
     The HDD group installation drawer  19  can be withdrawn smoothly toward the front side (in a direction that is the reverse of the depth direction of the basic enclosure  1 A) and can be pushed in the depth direction by means of the drawer slide mechanism  27  mentioned earlier. In a case where an optional HDD  23  is mounted, the user withdraws the HDD group installation drawer  19  and inserts the HDD  23  downward vertically from above into an optional HDD slot  83  among a plurality of HDD slots  83 . The user also withdraws the HDD group installation drawer  19  and removes the HDD  23  in the optional HDD slot  83  upward vertically from below when an optional HDD  23  is removed from the basic enclosure  1 A. Such installation or removal of the HDD  23  can be performed even when the power supply of the controller units  37 A,  37 B, HDD  23 , and so forth is ON. That is, hot swapping of the HDD  23  is possible. 
     As shown in  FIGS. 5A and 5B , the HDD group installation drawer  19  is withdrawn separately from the back plane  17 . In other words, the rear side parts (the back plane  17  and the parts that are further in the depth direction than the back plane  17 ) are fixed. Only the HDD group installation drawer  19  slides. A cable  63 , which provides an electrical connection between the connector  47  of the HDD group installation drawer  19  and the front-side connector  48  of the back plane  17 , is provided. The cable  63  is a flexible-film-like cable, for example. As a result, in a state where the HDD group installation drawer  19  is completely housed in the basic enclosure  1 A, for example, the cable  63  is housed in the space between the HDD group installation drawer  19  and the bottom of the basic enclosure  1 A (a space with the height of the rail  25 , for example). Further, it is considered advantageous from the point of view of reducing the number of parts and to make is easy to exchange the rear parts if, as shown in  FIGS. 5A and 5B , the whole of the HDD group installation drawer  19  slides and the rear parts are fixed. Further, an extendable rail  25  may be provided in place of the cable  63  in a constitution in which only the HDD group installation drawer  19  is withdrawn. In such a case, when the HDD group installation drawer  19  is withdrawn, the rail  25  extends (that is, grows longer) in a direction that is the reverse of the depth direction and contracts (that is, grows shorter) in the depth direction when the HDD group installation drawer  19  is pushed in. 
     The method of withdrawing the HDD group installation drawer  19  is not limited to the method shown in  FIGS. 5A and 5B . A variety of variations may be considered. A variety of variations will be described in the second embodiment example (described subsequently). 
     Further, in the first embodiment, as mentioned earlier, a plurality of HDDs  23  is arranged in the directions of two dimensions in the HDD group installation drawer  19  and this plurality of HDDs  23  is arranged at equal intervals in the row direction (that is, in a direction that is orthogonal to the depth direction and not in a vertical direction). As a result, it is possible to make uniform the air stream flowing through the long space (referred to as an ‘intercolumn path’ hereinafter) in the depth direction that arises between one column and an adjacent column. Further, this fact is described in detail below. 
       FIG. 6A  shows the appearance of the flow of an air stream when an HDD  23  is mounted in all the HDD mounting portions  31  of the HDD group installation drawer  19 .  FIG. 6B  is an enlarged view of an intercolumn path in  FIG. 6A . 
     When an HDD  23  is mounted in all the HDD mounting portions  31  of the HDD group installation drawer  19 , the widths of all of the intercolumn paths  91  are the same length. Further, the width of the respective intercolumn paths (that is, the paths for the air stream that flows from the front side to the rear side)  91  is constant from the end of the front side (front end) to the end of the rear side (rear end). Hence, the volume and velocity of the air stream flowing through the intercolumn paths  91  is the same for all the intercolumn paths  91 . Hence, the cooling design is straightforward. For example, an air stream flowing through an intercolumn path  91  is hotter on the rear-end side of the intercolumn path  91  than on the front-end side thereof due to the heat generated by the HDD  23 . The cooling design can be executed based on that fact. 
       FIG. 7A  shows an example of one variation on the cooling design. 
     For example, a SAS-HDD reaches a high temperature more readily than a SATA-HDD (in other words, readily emits more heat). This is thought to be because the number of rotations per unit of time is higher for a SAS-HDD than for a SATA-HDD, for example. For this reason, when a SAS-HDD is disposed on the front side of the basic enclosure  1 A and a SATA-HDD is disposed further toward the rear side, the SATA-HDD disposed on the rear side receives hot air. As a result, the cooling efficiency of a SATA-HDD is poor. 
     Therefore, as shown in  FIG. 7A , SATA-HDDs (SATA-HDDs arranged in the row direction, for example) are mounted on the front side of the HDD group installation drawer  19  (that is, on the upstream-side in the air flow direction) and SAS-HDDs (SAS-HDDs arranged in the row direction, for example) are mounted on the rear side of the HDD group installation drawer  19  (that is, on the downstream side in the air flow direction). More specifically, SATA-HDDs are mounted throughout the one or more rows on the front side, and SAS-HDDs are mounted throughout the one or more rows on the rear side, as shown in  FIG. 7A , for example. As a result, the SATA-HDDs receive an air stream at a lower temperature than when the SAS-HDDs are disposed closer to the front side than the SATA-HDDs (more specifically, an air stream that has not been deprived of the heat of any HDD or that has the heat taken from the SATA-HDDs that are at a lower temperature than the SAS-HDDs). Hence, in comparison with a case where the SAS-HDDs are disposed closer to the front side than the SATA-HDDs, the cooling efficiency of the SATA-HDDs can be raised. Further, the air stream received by the SAS-HDDs is at a higher temperature than in a case where the SAS-HDDs are disposed closer to the front side than the SATA-HDDs. However, because the air stream is at a lower temperature than the temperature of the SAS-HDD, cooling of the SAS-HDDs can also be performed. That is, in comparison with a case where the SAS-HDDs are disposed closer to the front side than the SATA-HDDs, the overall cooling efficiency can be increased. 
       FIG. 7B  shows an example of another variation on the cooling design. 
     In a case where HDDs  23  are mounted on all the HDD mounting portions  31  of the HDD group installation drawer  19 , the widths of all the intercolumn paths  91  are the same length as mentioned earlier. Consequently, the volume and velocity of the air stream flowing through the intercolumn paths  91  is also the same for the other intercolumn paths  91 . However, HDD  23  may not be installed in all the HDD mounting portions  31 . In this case, the width at a certain point of a certain intercolumn path  91  differs from the width at other points. Hence, the possibility that the cooling efficiency will drop exists. 
     Therefore, as shown in  FIG. 7B , a dummy HDD  94  is inserted in a vacant HDD slot  83 F in which an HDD  23  has not been inserted. Here, the dummy HDD  94  may be any dummy as long as same is a solid body with the same volume and shape as the HDD  23 . For example, the dummy HDD  94  may be a different box with the same volume and shape as the HDD  23 . 
     Therefore, because the width at any point of the intercolumn path  91  is rendered the same by inserting the dummy HDDs  94  in vacant HDD slots  83 F, a drop in cooling efficiency can be prevented. 
     Further, the dummy HDDs  94  may be located anywhere as long as same are inserted in vacant HDD slots  83 F. For example, dummy HDDs  94  may be distributed equally throughout the HDD group installation drawer  19  or may be arranged concentrated in a certain location. More specifically, for example, HDDs (SATA-HDDs, for example) may be arranged in the first row closest to the front face (the row closest to the air stream opening), HDDs (SAS-HDDs, for example) may be arranged in the mth row (m=4, for example) that is furthest from the front face, and dummy HDDs  94  may be installed throughout at least one row between the first and mth rows. 
     Further, when an HDD is removed from a certain row and a first vacant HDD slot  83 F becomes available, a second vacant HDD slot  83 F may be prepared by removing an HDD from an HDD slot  83  in another row and then inserting the HDD thus removed into the first vacant HDD slot  83 F, for example. A dummy HDD  94  may be inserted into the second vacant HDD slot  83 F thus prepared to replace the HDD  23 . That is, the dispositional relationship between the HDD  23  and the dummy HDD  94  can be chosen by the user. 
     Further, as mentioned earlier, a door, which is in a closed state when the HDD  23  is pressed into the opening of the HDD slot  83  to open same and there is no HDD  23  and/or when the HDD  23  is inserted completely, for example, may be provided. In other words, a shutter, which prevents the opening of the HDD slot  83 F from being opened when an HDD  23  is not inserted may be provided at the opening of the HDD slot  83 . 
     Further, the state display lamps  81 A and  81 B corresponding with each of the HDD slots  83  (see  FIGS. 2B and 4A ) execute a display that corresponds with a case where a dummy HDD  94  has been inserted into the HDD slots  83  (that is, the user is notified that the dummy HDD  94  has been inserted). Hard control can also be implemented by means of computer control by the controller units  37 A and  37 B. For example, the face opposite the HDD group mounting substrate  21  of the dummy HDD  94  comprises a projection of a certain length and the HDD group mounting substrate  21  comprises a contact point for detecting contact with the projection. When the dummy HDD  94  is inserted and the projection makes contact with the contact point, the state display lamps  81 A and  81 B may execute a corresponding display (the flicker of a green lamp, for example). 
     In addition, the state display lamps  81 A and  81 B corresponding with each HDD slot  83  may execute a display that corresponds with a status other than a status that indicates that the dummy HDD  94  has been inserted. For example, when the HDD  23  is inserted and an error is not detected with the HDD  23 , the controller units  37 A and  37 B may cause the state display lamps  81 A and  81 B corresponding with the HDD slot  83  into which the HDD  23  has been inserted to execute a display (a red lamp lights up, for example) that represents an active status. Further, in a case where the HDD  23  has been inserted and an error with the HDD  23  is detected, for example, the controller units  37 A and  37 B may cause the state display lamps  81 A and  81 B corresponding with the HDD slot  83  into which the HDD  23  has been inserted to execute a display indicating a fault (green lamp is goes out, for example). Further, upon detecting that the lock control button  35  has been pressed, the controller units  37 A and  37 B may render the display of the state display lamps  81 A and  81 B of the column corresponding with the lock control button  35  an aspect that corresponds with such detection (a green lamp and red lamp may light up alternately, for example). As a result, the user learns that an HDD  23  has been removed from an HDD slot  83  belonging to any column and hence the erroneous removal of an HDD  23  that is not intended by the user can be prevented. Further, when it is detected that the eject button  85  of a certain HDD slot  83  belonging to a column corresponding with the lock control button  35  has been pressed, the controller units  37 A and  37 B may cause the state display lamps  81 A and  81 B corresponding with the certain HDD slot  83  to execute a display (flicker of a green lamp, for example) that signifies that removal of the HDD  23  is allowed. 
     Variations in the cooling design were described above. However, the details of the cooling design are not limited to the examples above. Rather, a variety of other variations may be considered. Such a variation will be described by means of a subsequent fourth embodiment example. 
       FIG. 8  shows an example of the constitution within the controller units  37 A and  37 B, an example of the constitution within the switch enclosure  3 , an example of the constitution within the expansion enclosure  1 B, and an example of the constitution of the connection between the controller units  37 A,  37 B and the expansion enclosure  1 B. 
     The controller units  37 A and  37 B are provided with controller circuits  101 A and  101 B respectively. Because the constitution of the controller circuits  101 A and  101 B is the same, when this is described taking the controller circuit  101 A as a representative example, the controller circuit  101 A comprises a CPU  105 , a bridge circuit (abbreviated to ‘Bridge’ in  FIG. 8 )  107 , a Fibre Channel Interface circuit (abbreviated to ‘IF-FC’ in  FIG. 8 )  111 , a cache memory (abbreviated to ‘Cache’ in  FIG. 8 )  114 , a data transfer control circuit (abbreviated to ‘D_CTL’) in  FIG. 8 )  113 , a SAS interface circuit (abbreviated to ‘IF-SAS’ in  FIG. 8 )  115 , a fanout expander (abbreviated to ‘F-Exp’ in  FIG. 8 )  116 , and a main memory (abbreviated to simply ‘memory’ in  FIG. 8 )  109 . The CPU  105  performs centralized control of the controller circuit  101 A (control of each element of the controller circuit, for example). The bridge circuit  107  controls the connection between each of the elements of the controller circuit that are connected to the Bridge  107  (CPU  105 , main memory  109 , and so forth) and the elements of another controller circuit that are connected to the Bridge  107 . The IF-FC  111  is a circuit in which a chip for processing the FC (Fibre Channel) protocol is installed and to which a host device (not shown) is connected via a SAN (Storage Area Network), for example. The Cache  114  is a memory for the temporary storage of data that is written to the HDD  23 , data that is read from the HDD  23 , and so forth. The IF-FC  111 , cache memory (abbreviated to ‘Cache’ in  FIG. 8 )  114 , and D_CTL  113  control data transfers between each of the elements of the controller circuit that are connected to the D_CTL  113  and the elements of another controller circuit that are connected to the D_CTL  113 . The IF-SAS  115  is a circuit in which a chip that processes the SAS protocol is installed and which communicates with a SAS-HDD  23 S via an F-Exp  116 . The F-Exp  116  is one type of switch circuit that comprises j (j is an integer of one or more) first ports that are connected to the IF-SAS  115  and k (k&gt;j) second ports that are connected to a plurality of rear-side connectors  43  of the controller unit  37 A. The memory  109  may be a ROM for storing a computer program that is read to the CPU  105  or may be RAM that comprises a work region for the CPU  105 . The front-side connector  41  may be connected to the Bridge  107  and may be connected to the front-side connector  41  of the controller unit  37 B via a path  106 . As a result, the controller circuit  101 A is able to access the other controller circuit  101 B. For example, the controller circuit  101 A may write data read from the HDD  23  to the Cache  114  and may write data to the Cache in the controller circuit  101 B via the path  106 . 
     One or more (two, for example) switch devices  118  may be provided in the switch enclosure  3 . Each of the switch devices  118  may be provided with a plurality of edge expanders (abbreviated to ‘E-Exp’ in  FIG. 8 )  117 . The E-Exp  117  is one kind of switch circuit that comprises p (p is an integer of one or more) first ports that are connected to the controller unit  37 A or  37 B and q (q&gt;p) second ports that are connected to one or more expansion enclosures  1 B. 
     The expansion enclosure  1 B comprises a disk connection portion  120 , a plurality of HDD  23 , and a back plane (abbreviated to ‘BP’ hereinafter)  121  that connects the disk connection portion  120  and the plurality of HDD  23 . 
     A switch portion  119  in the expansion enclosure of the disk connection portion  120  implements the same processing as the switch portion  118 . 
     A BP  121  has a first port  126 , which is connected to the switch portion  119  in the expansion enclosure, and a second port  128 , which is connected to the HDD  23 . A second port set  128  of the BP  121  has two second ports (not shown) and, connected to these two second ports via the switch device  118  and switch portion  119  in the expansion enclosure are the two controller circuits  101 A and  101 B respectively. In cases where the HDDs  23  connected to the two second ports are SAS-HDD  23 S, the SAS-HDDs  23 S have two connectors  32 . Hence, the SAS-HDDs  23 S are connected directly to the two second ports (to one second port set  128 ). On the other hand, in cases where the HDDs  23  connected to the two second ports are SATA-HDDs  23 A, the SATA-HDDs  23 A have one connector  32 , and hence the SATA-HDDs  23 A are connected via a path switch (abbreviated to ‘PS’ hereinafter)  123  to two second ports (to one second port set  128 ). The PS  123  is a device (a chip, for example) that comprises two first ports that are connected to the second port set  128  and one second port that is connected to the SATA-HDD  23 A. 
     As a result of the above constitution, the two controller circuits  101 A,  101 B are able to access each SATA-HDD  23 A and each SAS-HDD  23 S. That is, the two controller circuits  101 A and  101 B are able to manage all of the HDDs  23  provided in the storage subsystem  1  individually. Further, the constitution of the connection between the HDDs  23  in the expansion enclosure  1 B and the BP  121  can be applied to the constitution of the connection between the HDDs  23  in the basic enclosure  1 A and the BP  17 . Further, the constitution within the controller units  37 A and  37 B, the constitution within the SW enclosure  3 , the constitution within the expansion enclosure  1 B, and the constitution of the connection between the controller units  37 A and  37 B and each HDD  23  are not limited to the above example(s). Rather, several variations may be considered. Such a variation will be described by means of a fifth embodiment example (described subsequently). 
       FIG. 9  shows an example of the constitution of the network in a case where the storage subsystem  1  is connected to a communication network. 
     The controller circuit  101 A (and/or  101 B) is provided with a management I/F  205  that functions as an interface for a management terminal  203 . The management terminal  203  is connected to the management I/F  205  via a communication network (a LAN, for example)  204 . The management terminal  203  is a computer machine (a personal computer or server machine, for example) that comprises a CPU, memory, and so forth. The CPU  105  is able to communicate with the management terminal  203  via the management I/F  205 . Further, the management terminal  203  may be connected to the management I/F  205  by means of a single cable. 
     A logical storage device (‘logical volume’ hereinafter)  213  is prepared on one or more HDDs  23 . A group of one or more HDDs  23  that comprise the logical volume  213  is referred to as a ‘parity group’ hereinbelow. Further, the types of the one or more HDDs that are present in one parity group may all be the same or may be different. If the types of the one or more HDDs are all the same, the reliability of the constitutional elements (HDD) of the parity group is the same. If the types differ, the constitution and disposition of the HDD are afforded greater freedom. 
     A host device  201  is connected to the IF-FC  111  of the controller circuit  101 A (and/or  101 B) via a communication network (a SAN, for example)  202 . In this case, the IF-FC  111  functions as an interface circuit for the host device. The CPU  105  receives a write command or read command from the host device  201  via the IF-FC  111  and executes write processing or read processing in response to this write command or read command. In the case of write processing, for example, the CPU  105  receives data from the host device  201 , writes the received data to the Cache  114 , and then reads this data from the Cache  114  before storing same in the logical volume  213  (in other words, one or more HDDs  23 ). In the read processing, the CPU  105  writes the data read from the HDD  23  to the Cache  114  and then reads the data from the Cache  114  before sending same to the host device  201 , for example. Further, the communication networks  202  and  204  may be the same communication networks. 
     Control information  211  is prepared in the main memory  109 . The control information  211  comprises data representing the relationship of correspondence between the HDDs  23  and the logical volume  213 . More specifically, for example, the control information  211  includes a volume identifier (a number, for example) for each logical volume  213 , information relating to the parity group constituting the logical volume  213  (information on each HDD  23 , for example (one example is the position, type and identifier thereof)), and the RAID level of the logical volume  213 . The control information  211  also includes information relating to a backup HDD (‘backup HDD’ hereinafter)  23  that does not constitute the logical volume  213  (the position, type and identifier (a WWN (World Wide Name), for example) thereof, for example). The CPU  105  is able to update the control information  211  in the main memory  109 . For example, upon detecting removal of an HDD  23 , the CPU  105  specifies the position at which the HDD  23  was removed (specifies the position based on which eject button  85  of which HDD slot  83  has been pressed, for example), and may then erase information on the HDD corresponding with the specified position from the control information  211 . Further, for example, in a case where the mounting of an HDD  23  has been detected, the CPU  105  may acquire information relating to the HDD  23  (the CPU  105  may send a predetermined command and receive information on the HDD  23  from the HDD  23  in response to the command, for example) and may include the acquired information in the control information  211 . Further, the control information  211  may be information that is inputted by the management terminal  203 . Further, the control information  211  may be updated by the management terminal  203 . Further, the information on each HDD  23  included in the control information  211  may include the status (‘active’, ‘faulty’, ‘maintainable’, ‘standby’, and ‘in-preparation’, for example) of each HDD  23  detected by the CPU  105 . Further, the type of each HDD  23  included in the control information  211  may be automatically detected by the CPU  105 . In cases where it is detected that the HDD has been accessed directly without passing via the PS above, for example, the CPU  105  may judge that the HDD is a SAS-HDD and, when it is detected that the HDD has been accessed via the PS, the CPU  105  may judge that the HDD is a SATA-HDD. Alternatively, the CPU  105  may send a predetermined command (a SCSI device discovery command, for example) to each HDD  23  and detect the type of each HDD  23  on the basis of the information that has been sent back from each HDD  23  in response to the command. 
     The CPU  105  is able to generate a GUI screen (illustrated below) on the basis of the control information  211  and display the GUI screen on the management terminal  203 . 
       FIG. 10A  shows an example of a GUI screen that is displayed on the management terminal  203  when a fault has not occurred. The HDD information that is displayed on the GUI screen is information on the HDDs that exist in a enclosure (basic enclosure  1 A, for example) that has been designated by the user. 
     A GUI screen  301  is divided into a first subscreen  301 A and a second subscreen  301 B. 
     The CPU  105  displays information on the logical volume  213  on the first subscreen  301 A. More specifically, for example, the CPU  105  displays one or more volume graphics  308  that correspond with one or more logical volumes  213  and, based on the control information  211 , displays an identifier for each logical volume  213 , information on the parity group constituting the logical volume  213 , and the RAID level of the logical volume  213  near each volume graphic  308 . Further, the CPU  105  also displays information on one or more backup HDD  23  (the type and identifier, for example) on the first subscreen  301 A. Further, the CPU  105  may display the type (SAS or SATA, for example) of the HDD  23  corresponding with the HDD graphic  303  within the HDD graphic  303 . 
     In addition, the CPU  105  displays information on a plurality of HDD  23  that are installed in the storage subsystem  1  on the second subscreen  301 B on the basis of the control information  211 . For example, the CPU  105  displays information on all the HDDs  23  that exist in each of the enclosures  1 A,  1 B or on the HDDs  23  that exist in a enclosure that is specified by the user. The second subscreen  301 B shown in  FIG. 10A  displays information on the HDDs  23  that exist in the basic enclosure  1 A. 
     For example, the CPU  105  prepares locations on the second subscreen  301 B (‘HDD display locations’ hereinbelow) that correspond with a plurality of positions (positions of a plurality of HDD mounting portions  31 ) in the HDD group installation drawer  19  and displays a graphic representing an HDD (‘HDD graphic’ hereinbelow)  303  at these HDD display locations. Thereupon, when the number of rows is four, for example, the CPU  105  displays the number of the row closest to the front face of the basic enclosure  1 A (or expansion enclosure  1 B) as ‘00’ and displays the number of the row furthest from the front face of the basic enclosure  1 A (or expansion enclosure  1 B) as ‘03’. Further, the CPU  105  displays information on what kind of logical volume is constituted by an HDD in a particular position (for example, one or more HDD graphics corresponding with one or more HDDs comprising a logical volume are framed and the identifier of the logical volume is displayed near the frame). Further, the CPU  105  may not display HDD graphics in positions in which HDDs do not exist or may display a different form of display from the display form (red, for example) of the HDD graphics in positions where HDDs exist. In addition, in cases where a certain HDD graphic is designated by the user (when clicked on with a mouse, for example) the CPU  105  may extract information on the HDD corresponding with the HDD graphic (type, identifier, status and logical volume identifier, for example) from the control information  211  and then display this information. Further, the illustrated status known as ‘standby’ is a backup HDD status that signifies that one of the HDDs constituting the logical volume can be integrated, for example. 
       FIG. 10B  shows an example of a GUI screen that is displayed on the management terminal  203  when a fault has occurred.  FIG. 10B  is for a case where a fault has arisen with a certain HDD  23  that comprises a logical volume with the logical volume identifier ‘#2’. 
     Upon detecting that a fault has occurred in a certain HDD  23  (in a case where no response is received from the HDD  23  even when access is retried a predetermined number of times, for example), the CPU  105  references the control information  211  to specify the identifier of the logical volume  213  that comprises the HDD (‘faulty HDD’ hereinbelow)  23  in which a fault has been detected. Further, the CPU  105  highlights graphic  308  of the logical volume  213  with a specified identifier ‘#2’ on the first subscreen  301 A (the graphic  308  is circled by a bomb mark  309 , for example). The CPU  105  also displays ‘redundant’ as the status of the logical volume  213  with the identifier “#2”. 
     Further, the CPU  105  highlights an HDD graphic  303 A at the HDD display location corresponding with the position of the faulty HDD  23  on the second subscreen  301 B. Further, the CPU  105  displays information on the faulty HDD  23  (the type, ‘faulty’ status, and logical volume identifier, for example) so that this information is associated with the HDD graphic  303 A. 
     Thus, the CPU  105  is able to inform the user of the nature of the fault when a fault has occurred in the storage subsystem  1 . The CPU  105  may detect a recovery-in-progress status and display this status on the second subscreen  301 B. For example, when the CPU  105  receives an instruction from the management terminal  203  to include ‘standby’ status HDDs in one of the parity groups comprising a certain logical volume, the CPU  105  includes these HDDs in one of the parity groups and may accordingly execute HDD integration processing, which increases the storage capacity of the certain logical volume. Further, here, if the HDD integration processing has started, the CPU  105  may switch the status of the HDD to be processed from ‘standby’ to ‘in preparation’ and, as shown in  FIG. 10B , display the ‘in preparation’ status so that same is associated with an HDD GRAPHIC  303 B that corresponds with the HDD to be processed. Further, if the HDD integration processing is complete, the CPU  105  may switch the status of the HDD to be processed from ‘in preparation’ to ‘active’ and display the ‘active’ status so that same is associated with the HDD GRAPHIC  303 B corresponding with the HDD to be processed. 
       FIG. 11  shows an example of the flow of processing that is executed up until a fault is recovered after a fault occurs with an HDD. 
     When a fault has occurred with a certain HDD  23 , the fault is detected by the CPU  105 . Thereupon, the CPU  105  causes the first state display lamps  81 A and  81 B (see  FIG. 4A ) that correspond with the faulty HDD  23  to execute a display that signifies that a fault has occurred. The CPU  105  also causes the second state display lamp  33  corresponding with the column to which the faulty HDD  23  belongs to execute a display that signifies that a fault has occurred in that column (step S 100 ) The CPU  105  displays a GUI screen  301 , on which information on the HDD  23  (the position, status, and the like, for example) in which the fault has occurred is placed, on the management terminal  203  on the basis of the control information  211  (S 200 ) A user  401  of the management terminal  203  views the GUI screen  301  displayed on the management terminal  203 , examines the strategy regarding which settings are to be made for the storage subsystem  1 , and then uses the management terminal  203  to make settings for the storage subsystem  1  based on the examination results (S 300 , S 301 ). The CPU  105  of the storage subsystem  1  analyses the content of the settings and then executes processing based on the results of the analysis (S 400 ). 
     A specific example of S 301  and S 400  will be provided below. 
     In S 301 , the management terminal  203  releases the lock on the faulty HDD  23  in accordance with an operation by the user  401  and then inputs the settings to include the backup HDD selected by the user (‘selected backup HDD’ hereinbelow) in the parity group that includes the faulty HDD  23  to the storage subsystem  1 . 
     The CPU  105  of the storage subsystem  1  then releases the lock on the faulty HDD  23  by controlling the lock mechanism  87  of the faulty HDD  23  in accordance with the settings thus inputted (S 401 ). Thereupon, the CPU  105  may change the status of the faulty HDD  23  from ‘faulty’ to ‘maintainable’ and display the changed status on the management terminal  203  so that the changed status is associated with the HDD graphic of the faulty HDD  23 . Further, the CPU  105  may also release the lock on all of the HDD  23  that constitute the parity group (hereinafter ‘faulty group’) that includes the faulty HDD  23 . This serves to enable HDD replacement in parity group units. Further, the CPU  105  may not access the HDD  23  whose lock has been released (for example, in a case where a write command or read command for the logical volume that the HDD  23  comprises has been received, notification to the effect that access is now prohibited may be sent back to the host device). Further, the CPU  105  may issue notification that the lock control button  35  corresponding with the column of the faulty HDD  23  has been pressed (the display of the second state display lamp  33  is controlled to provide notification to that effect, for example) and the lock of the faulty HDD  23  may be released after it is detected that the lock control button  35  has been pressed by way of response. 
     Further, the CPU  105  turns OFF the power supply of the faulty HDD  23  in accordance with the inputted settings (S 402 ). The CPU  105  may then turn OFF the power supply of all the HDD  23  constituting the faulty group. As the method for turning OFF the power supply, a method in which, in a case where the HDD group mounting substrate  21  is provided with a power supply circuit  404  that supplies electrical power to the HDD  23 , the CPU  105  turns OFF the power supply of the HDD  23  by sending a power supply turn-OFF command to the power supply circuit  404 , may be considered. 
     Further, the CPU  105  may cause the first state display lamps  81 A and  81 B corresponding with the faulty HDD  23  to execute a display to the effect that the lock on the faulty HDD  23  has been released (S 403 ). 
     The CPU  105  also executes processing to include the selected backup HDD  23  in the parity group in place of the faulty HDD  23  (S 404 ). For example, the CPU  105  updates the control information  211  to information that includes content indicating the fact that the selected backup HDD  23  is included in the faulty group in place of the faulty HDD  23 . 
     The first embodiment example was described above. Further, in the first embodiment example, instead of withdrawing or pushing in the HDD group installation drawer  19  and mounting or removing the HDD  23 , the constitution is such that the upper faces of the enclosures  1 A and  1 B open and close such that mounting or removal of the HDD  23  may be performed by opening these upper faces. The upper faces of the enclosures  1 A and  1 B may be opened and closed by turning the enclosures  1 A,  1 B with a certain edge serving as the axis or may be opened and closed by sliding along the upper faces in the directions of two dimensions, for example. According to the above first embodiment example hereinabove, a plurality of HDDs  23  may be arranged upright in the depth direction of the enclosures  1 A and  1 B. As a result, HDDs  23  can be provided at a high density within a fixed space. As a result, a higher performance and capacity for the storage subsystem  1  are also possible. 
     Furthermore, according to the above first embodiment example, a plurality of HDDs  23  is arranged at equal intervals in the row direction that is orthogonal to the direction in which the cooling air stream flows. As a result, because all the widths of the intercolumn paths  91  are then the same, the volume, velocity, and so forth, of the cooling air stream passing through each intercolumn path  91  can be made uniform. 
     Further, according to the above first embodiment example, the lock control button  35  is provided for each column constituted by two or more HDDs and, unless the lock control button  35  corresponding with the desired column is pressed, the HDDs  23  belonging to that column cannot be removed. As a result, it is possible to prevent erroneous removal of HDDs that are present in the rows adjacent to an HDD that is to be removed and therefore maintenance work on the storage subsystem  1  can be performed accurately. 
     Further, several variations may be considered based on a variety of facts in the first embodiment example above. Embodiment examples for which a variety of variations has been adopted will be described hereinbelow as other embodiment examples. Further, in the following description, descriptions of points that are also common to the first embodiment example are omitted or simplified. Points of difference from the first embodiment example will mainly be described. 
     Second Embodiment Example 
     In the second embodiment, the method of withdrawing the HDD group installation drawer  19  differs from that of the first embodiment example. 
     Withdrawal methods include, for example, a method (referred to as the ‘integrated withdrawal method’ hereinafter) in which the HDD group installation drawer  19  is withdrawn integrally with the rear-side parts (the back plane  17  and the parts that exist further in the depth direction than the back plane  17 ) and a method (referred to as the ‘separate withdrawal method’ hereinafter) in which the HDD group installation drawer  19  is withdrawn separately from the back plane  17 . Variations on each withdrawal method will be described below. 
     (A) Integrated Withdrawal Method 
       FIG. 12A  shows a first variation on the integrated withdrawal method. 
     According to the first variation, a cable (hereinafter ‘external cable’)  77 , which is connected to the rear-side connector  43  of the power supply unit  39 , controller units  37 A,  37 B, and so forth, is drawn toward the front side and introduced to the basic enclosure  1 A in accordance with the sliding action of the HDD group installation drawer  19  and, when the HDD group installation drawer  19  is pushed toward the rear side, the cable  77  is made to exit the basic enclosure  1 A. 
     According to the first variation, the rear face of the basic enclosure  1 A is open. It is therefore, easy to exchange the power supply unit  39  and controller units  37 A,  37 B, and so forth. 
       FIG. 12B  shows a second variation on the integrated withdrawal method from which an illustration of the slide mechanism  27  has been omitted. 
     According to this second variation, a boundary substrate  79  is provided near the rear face of the basic enclosure  1 A. The boundary substrate  79  is a printed substrate, for example, and the space near the rear face of the basic enclosure  1 A is divided into the front side and rear side of the basic enclosure  1 A. The boundary substrate  79  comprises, on the rear face thereof, a connector  81  for connecting the external cable  77  and comprises, on the front side, a connector  80  that is connected to the rear-side connector  43  of the power supply unit or the like via an internal cable  78 . The position of the boundary substrate  79  is fixed and the boundary substrate  79  does not move in accordance with the insertion and withdrawal of the HDD group installation drawer  19 . 
     Further, according to the second variation, the boundary substrate  79 , power supply unit  39 , and controller units  37 A,  37 B, and so forth are constituted as a single module, meaning that the power supply unit  39  and controller units  37 A,  37 B, and so forth may be exchanged for another power supply unit  39 , and controller units  37 A,  37 B, and so forth by exchanging the module. 
       FIG. 12C  shows a third variation on the integrated withdrawal method from which an illustration of the slide mechanism  27  has been omitted. 
     The third variation is a modified example of the second variation above. That is, a new connector  501  is provided on the rear face of the back plane  17  and the front-face connector  80  of the boundary substrate  79  is connected to the connector  501  via the internal cable  78 . The internal cable  78  is stored in the space between the controller unit  37 B and the bottom of the basic enclosure  1 A. The internal cable  78  is a flexible-film-like cable, for example. 
     According to the third variation, the number of parts can be reduced. For example, the number of internal cables  78 , the number of front-side connectors  80  of the boundary substrate  79 , and the number of rear-side connectors  43  of the respective units  39 ,  37 A and  37 B can be reduced. Further, it is understood that the constitution of the printed wiring of the back plane  17  in the third variation differs from that of the first embodiment example. 
     (B) Separate Withdrawal Method 
       FIG. 13A  provides an outline of one variation on the separate withdrawal method and  FIG. 13B  serves to illustrate the variation in detail. 
     According to the variation on the separate withdrawal method shown in  FIGS. 13A and 13B , the HDD group installation drawer  19  is constituted by n subdrawers  19 S. The value of n is an integer of two or more and a value less than the number of columns of the HDD  23 . Further, in this second embodiment example, n is the same number as the number of columns of the HDD  23 . 
     HDDs  23  that constitute one or more columns (one column in this embodiment example) are mounted in one subdrawer  19 S. Each subdrawer  19 S may have the constitution of the HDD group installation drawer  19 . For example, each subdrawer  19 S may comprise, at the bottom thereof, an HDD group installation sub-substrate for mounting the HDDs that belong to the column corresponding with the subdrawer  19 S. 
     Each subdrawer  19 S is withdrawn separately from the back plane  17  as shown in  FIGS. 13A and 13B . That is, although there is no special illustration, each subdrawer  19 S is provided with a slide mechanism for sliding the subdrawer  19 S toward the front side and toward the rear side. As per the first embodiment example, this slide mechanism may be constituted by a rail and guide rollers. 
     The connector  47  of each subdrawer  19 S is connected electrically to the HDD  23  that is mounted in the subdrawer  19 S. Further, the connector  47  of each subdrawer  19 S is connected to the front-side connector  48  of the back plane  17  via a cable  508 . The cable  508  is a flexible-film-like cable, for example. As a result, in a state where the subdrawer  19 S is completely housed within the basic enclosure  1 A, the cable  508  is housed in a space between the subdrawer  19 S and the bottom of the basic enclosure  1 A (a space that has the height of the rail  25 , for example. Further, in this variation, an extendable rail may be provided instead of providing the cable  508 . In such a case, when the subdrawer  19  is withdrawn, the rail extends (that is, grows longer) toward the front side and contracts (that is, grows shorter) toward the rear side when the subdrawer  19  is pushed in. 
     Third Embodiment Example 
     In the third embodiment example, the removal method for the HDD  23  differs from that of the first embodiment example. 
       FIG. 14A  shows a first variation on the removal method for the HDD  23 . 
     In the first variation, an elastic body (a spring, for example)  511  whose height runs vertically is provided between the HDD mounting portion  31  and HDD group mounting substrate  21 . In this first variation, the mounting and removal of the HDD  23  is executed according to the following processing flow. 
     That is, the HDD  23  is pressed vertically downward for insertion into the HDD slot  83  and, when the HDD  23  no longer extends after the elastic body  511  has collapsed according to a certain measure, the elastic body  511  returns to and stops in a position below the original height thereof such that the upper face  23 U of the HDD  23  is then located in the same position as the face of the HDD slot  83  or in a position below same. The HDD  23  thus enters a mounted state. 
     Thereafter, when the upper face  23 U of the HDD  23  is pressed vertically downward and the elastic body  511  is collapsed according to a certain measure, the elastic body  511  returns to the original height and the upper face  23 U of the HDD  23  is then located in a plane above the face of the HDD slot  83 . The HDD  23  thus enters a removable state. 
       FIG. 14B  shows a second variation on the removal method for the HDD  23 . 
     According to the second variation, a handle  512  is provided on the upper face of the HDD  23  (or of the canister housing the HDD  23 ). The user holds the handle  512  and is able to remove the HDD  23  by drawing the handle  512  upward in a vertical direction. 
     Further, the handle  512  may be mounted detachably on the HDD  23  (or the canister housing the HDD  23 ). Further, the shape of the handle  512  may be any shape as long as same can be gripped by hand. Further, in place of the handle  512 , a tool for pulling out the HDD  23  may be provided by using any means (through engagement or hooking, for example) may be provided. 
     Fourth Embodiment Example 
     In a fourth embodiment example, the cooling design of the HDD  23  differs from that of the first embodiment example. That is, according to the first embodiment example, the volume (or velocity) of the air stream flowing through the intercolumn paths  91  is substantially uniform over the interval from the inlet to the outlet thereof but is not uniform in the fourth embodiment example. More specifically, for example, according to the fourth embodiment example, the flow path of the air stream narrows from the front side toward the rear side (that is, as progress is made movement in the depth direction). As a result, the air stream velocity increases in moving toward the rear side and hence the cooling efficiency of the HDDs  23  that exist on the rear side can be improved. 
       FIG. 15A  shows a first variation on the cooling design for increasing the velocity of the cooling air stream on the rear side. 
     According to the first variation, a dividing member  564  that is long in the depth direction is disposed in the intercolumn path  91 . The dividing member  564  splits the intercolumn path  91  into two subpaths  91 S,  91 S from a point that is a predetermined distance apart from the opening of the intercolumn path  91  (close to the second row, for example) and the width of the subpath  91 S narrows in the depth direction. The dividing member  564  has a narrow width at the leading end (on the side opposite the depth direction) and widens at the trailing end (on the depth direction side), for example, and is a member with the same height as the height of the intercolumn path  91 . Because this dividing member  564  is disposed with the leading edge facing toward the front face and the rear edge facing toward the rear face, the intercolumn path  91  is divided into two subpaths  91 S,  91 S, the width (that is, the interval in the row direction) of each subpath  91 S growing narrower in the depth direction. As a result, the cooling air stream flowing through the intercolumn path  91  increases in velocity as same progresses in the depth direction, as indicated by the reference numerals  561  to  563 . 
     Further, various variations are possible for the shape of the dividing member  564 . The shape of the dividing member  564  may also be altered. For example, the dividing member  564  may be a plate in the form of a letter V (a metal plate, for example) such that the width between the leading edge and trailing edge can be adjusted by controlling the angle of the pointed end. 
       FIG. 15B  shows a second variation on the cooling design for increasing the velocity of the cooling air stream on the rear side. 
     In the case of the second variation, the column formed by two or more HDDs  23  is inclined in the depth direction so that the width between adjacent columns is narrower on the rear side than on the front side. Further, an adjustment member  565  for narrowing, in the depth direction, the width of the path of the cooling air stream flowing through the area adjacent to the first column (on the side where an adjacent column is not present), is provided in this adjacent area. 
     Further, in this second variation, the HDD group installation drawer  19  may have a constitution for adjusting the inclination of each column. For example, the HDD group mounting substrate  21  may be divided into a plurality of long paper-strip-like sub-substrates (sub-substrates that can comprise a column) oriented in the depth direction, and the inclination of each column may be adjusted by changing the inclination with respect to the depth direction of the sub-substrates. 
     Fifth Embodiment Example 
     In the fifth embodiment, at least one of the constitution within the controller units  37 A and  37 B, the constitution within the SW enclosure  3 , the constitution within the expansion enclosure  1 B and the constitution of the connection between the controller units  37 A and  37 B and each HDD  23  is different from that of the first embodiment example. 
       FIG. 16  shows one variation on the constitution within the expansion enclosure  1 B. 
     According to this variation, the SAS-HDD  23 S and SATA-HDD  23 A are not mixed within the expansion enclosure  1 B. Instead, only SAS-HDDs  23 S are present. That is, only SAS-HDDs  23 S are connected to the BP  121 . The SAS-HDDs  23 S have two ports as mentioned earlier and therefore can be connected directly to the BP  121  without the interposition of the PS  123 . 
     So too with this variation, the two controller circuits  101 A and  101 B are able to manage all of the HDDs  23  provided in the storage subsystem  1  individually. 
       FIG. 17  shows another variation on the constitution within the expansion enclosure  1 B. 
     According to this variation, the SAS-HDD  23 S and SATA-HDD  23 A are not mixed within the expansion enclosure  1 B. Instead, only SATA-HDDs  23 A are present. That is, only SATA-HDDs  23 A are connected to the BP  121 . The SATA-HDDs  23 A are equipped with only one port as mentioned earlier and are therefore connected to the BP  121  via the PS  123 . 
     So too with this variation, the two controller circuits  101 A and  101 B are able to manage all of the HDDs  23  provided in the storage subsystem  1  individually. 
       FIG. 18  shows yet another variation on the constitution within the expansion enclosure  1 B and one variation on the constitution of the connection between the controller units  37 A,  37 B and each HDD  23 .  FIG. 19  shows the constitution of the HDD group installation drawer in the expansion enclosure  1 B in detail. 
     According to  FIGS. 18 and 19 , the HDD group installation drawer  19  in the expansion enclosure  1 B comprises a plurality of subdrawers  19 S. That is, the plurality of HDD  23  in the expansion enclosure  1 B is logically divided into n columns (n is an integer of one or more). 
     The arrangement of the HDD  23  may be the same for the whole plurality of subdrawers  19 S or may be different. For example, as indicated by the reference number  19 SA in  FIG. 19 , all of the two or more HDDs  23  mounted in at least one subdrawer  19 S of the plurality of subdrawers  19 S may be a SAS-HDD  23 S. Further, as indicated by the reference number  19 SB, all of the two or more HDDs  23  mounted in at least one of the plurality of subdrawers  19 S may be a SATA-HDD  23 A. Further, two or more of the HDD  23  that are mounted in at least one subdrawer  19 S of the plurality of subdrawers  19 S may be a combination of SAS-HDDs  23 S and SATA-HDDs  23 A, for example. In this case, the SATA-HDD  23 A and SAS-HDD  23 S may be alternated moving in the direction from the front side of the expansion enclosure  1 B toward the rear side in at least one subdrawer  19 SC of the plurality of subdrawers  19 S. Further, at least one subdrawer  19 SD of the plurality of subdrawers  19 S may have a concentration of SATA-HDDs  23 A on the front side thereof and a concentration of SAS-HDDs  23 S on the rear side thereof. 
     According to  FIG. 18 , the controller circuits  101 A and  101 B are connected to the expansion enclosure  1 B without the interposition of the SW enclosure  3 . Further, the first controller circuit  101 A is connected to the HDDs  23  that exist on the rear side of each subdrawer  19 S but is not connected to the HDDs  23  that exist on the front side. The second controller circuit  101 B is connected to the HDDs  23  that exist on the front side of each subdrawer  19 S but is not connected to the rear-side HDDs  23  to which the first controller circuit  101 A is connected. 
     Several embodiments of the present invention were described hereinabove but the present invention is not limited to or by the above embodiments. A person skilled in the art is able to add to, take from or modify the constitution within the scope of the present invention. For example, because the illustrated flowchart is merely a flowchart to show clearly the processing flow, a person skilled in the art is able to switch, cancel or modify the steps easily so that an understanding and implementation of the invention are not impaired. 
     For example, among the connecting lines between the back plane  17  and HDD  23  or the connecting lines between the back plane  17  and each of the units  19 ,  37 A and  37 B, the power supply line may be constituted by wiring on the printed substrate and the signal line (the data transfer line, for example) may be constituted by a cable. 
     Moreover, a combination of columns that are constituted by SATA-HDD  23 A and columns that are constituted by SAS-HDD  23 S in the enclosures  1 A and  1 B are acceptable. In this case, the width of at least one intercolumn path  91  of the two intercolumn paths  91  adjacent to the column constituted by the SAS-HDDs  23 S is greater than the width of at least one intercolumn path  91  among the two intercolumn paths  91  adjacent to the column constituted by the SATA-HDD  23 A. This is because the SAS-HDD  23 S reaches a high temperature more readily than the SATA-HDD  23 A and therefore the volume of the air stream flowing adjacent to the column of SAS-HDDs  23 S is greater than the volume of the air stream flowing adjacent to the column of SATA-HDDs  23 A.  FIG. 20A  shows a specific example. In  FIG. 20 , columns constituted by SATA-HDDs  23 A and columns constituted by SAS-HDDs  23 S are arranged alternately. 
     Further, for example, control of the power supply of the plurality of HDDs  23  (to turn the power supply ON and OFF, for example) in each of the enclosures  1 A and  1 B may be performed for each HDD  23  or may be performed in n-column units (may be executed in units of the subdrawers  19 S, for example) or in x-row units (x is an integer of one or more), for example. 
     Moreover, for example, the types of HDDs  23  are not limited to at least one type of SAS and SATA. There may instead be different types (Fiber Channel, for example).