Patent Publication Number: US-8976530-B2

Title: Data storage apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to apparatus for storing data and supplying stored data. 
     2. Description of the Related Art 
     Data storage units are known that are configured for mass data storage or as RAID (redundant array of inexpensive disks) systems. Such storage units comprise of several disk drives and one or more control units for controlling data input to, and output from, the disk drives. The storage units may be located within a standard size rack unit, typically occupying 1 U to 6 U (1 to 6 rack units) of rack space, and may share the rack with other equipment, and possibly other such data storage units. 
     Occasionally it is necessary to access the disk drives, for example, for the purposes of replacement of a defective drive, or a drive needing to be upgraded. In some data storage units, access to the disk drives is made possible by the removal of a panel that forms part of the outer main housing of the storage unit. 
     In recent times it has become known to have a data storage unit in which the disk drives are arranged in several groups, each group supported on a separate support structure that provides electrical connections between the disks drives and a cable connected to the control unit. Each support structure may be slid forward from out of the outer main housing of the storage unit, while the support structure remains electrically connected by the cable to the control unit. 
     A first problem with such a data storage unit relates to a fan that is arranged to provide a flow of cooling air over the disk drives. During normal operation of the unit, the airflow from the fan is constrained by the outer main housing of the unit, such that it must flow over the disk drives. However, if one of the support structures is withdrawn from the outer main housing, for example to replace a defective disk drive, the airflow is no longer constrained and cooling of the disk drives becomes less effective. 
     A further problem with known data storage units that has been identified by the Applicant, relates to a lack of flexibility in the arrangement of disk drives within a data storage unit. For example, if a data storage unit having a particular number of disk drives of a particular type is being used, then the options for upgrading the storage capacity and/or performance of the unit are limited. 
     This latter problem is emphasised by a potential owner of a data storage unit purchasing a unit on his/her current requirements, current technology available, and current budget available. Consequently, a unit may be purchased that soon becomes inadequate and requires replacing. 
     BRIEF SUMMARY OF THE INVENTION 
     According to a first aspect of the present invention, there is provided apparatus for storing data and supplying stored data, said apparatus comprising: a main housing containing control circuitry for controlling signals to a plurality of disk drives, said main housing having a plurality of mounting mechanisms each configured to support a unit of a first type; a plurality of cables each having a first connector for connecting to a unit of said first type for providing signals from said control circuitry; a first unit, of a first type, having a plurality of second connectors each connected to a disk drive and a fan providing a flow of air for cooling the disk drives and said control circuitry, said first unit being connected to a first one of said cables and mounted in a first one of said mounting mechanisms such that said first unit is mounted to slide into said main housing; and a second unit, of a different second type, supported by a second one of said mounting mechanisms and mounted to slide into said main housing, said second unit being connected to a second one of said cables and having a fan providing a flow of air for cooling said control circuitry, wherein said second unit is removable from said main housing and replaceable by a unit of said first type. 
     According to a second aspect of the present invention, there is provided apparatus for storing data and supplying stored data, said apparatus comprising: a main housing; a plurality of units, each said unit having a plurality of connectors each connected to a data storage element, and each said unit being mounted to slide within said main housing such that each one of said units is slidable from out of said main housing to provide access to said data storage elements of said unit; and a fan configured to provide a flow of air for cooling the data storage elements, wherein each said unit comprises a channel such that said data storage elements are located within said channel, and said fan is arranged to provide a flow of air though said channel when said unit is located within said main housing and when said unit is slid out from within said main housing. 
     According to a third aspect of the present invention, there is provided apparatus for storing data and supplying stored data, said apparatus comprising: a main housing; an electric power supply located within said main housing; a panel located within said main housing, said panel supporting a plurality of electric connectors for connecting said power supply to other component parts of said apparatus; a plurality of units, each said unit comprising a plurality of data storage elements, each said unit being mounted to slide within said main housing such that each one of said units is slidable from out of said main housing to provide access to said data storage elements of said unit, and a plurality of cable modules, each module comprising a cable having a first connector at a first end configured to connect to one of said units and a second connector at a second end configured to connect with one of said electric connectors supported by said panel, and a moveable rigid structure supporting said second connector and providing a barrier preventing manual access to said panel thereby allowing said second connector to be disconnected by manually moving said rigid structure. 
     According to a fourth aspect of the present invention, there is provided electronic apparatus for mounting into a rack, said electronic apparatus comprising: a box; a wall element located within said box to define a first compartment to one side of said wall element and a second compartment to the other side of said wall element; a first unit mounted on slide rails within said first compartment; and a second unit mounted on slide rails within said second compartment, wherein said wall element supports a first slide rail supporting said first unit and a second support rail supporting said second unit, and said first slide rail is supported at a first height and said second slide rail is supported at a second different height, whereby the horizontal spacing between said first and second slide rails is reduced. 
     According to a fifth aspect of the present invention, there is provided electronic apparatus for mounting into a rack, said electronic apparatus comprising a box having a left side wall and a right side wall, wherein said side walls are formed from extruded metal defining a groove such that a groove extends along an outside surface of each of said left and right side walls for receiving a correspondingly shaped rail that is horizontally mounted within a rack. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  shows a data storage unit  101  embodying the present invention; 
         FIG. 2  shows a view of the front top and left side of the data storage unit  101 ; 
         FIG. 3  shows provides a view of the rear, top and left side of the data storage unit  101 ; 
         FIG. 4  shows a simplified cross-section view of the data storage unit  101 ; 
         FIG. 5  shows a further cross-section view of the data storage unit  101 ; 
         FIG. 6  shows a perspective view of the rear of the support unit  112  removed from the outer main housing  113 ; 
         FIG. 7  shows the cable module  212  removed from the data storage unit  101 ; 
         FIG. 8  shows the cable module  212  with the electric cables  412  in dotted outline only; 
         FIG. 9  shows a side view of the cable module  212 ; 
         FIGS. 10A ,  10 B and  10 C show the connection module  910  respectively in a perspective view, simplified bottom view, and simplified cross-sectional view; 
         FIG. 11  shows the data storage unit  101  mounted in the rack  102 , with support unit  112  pulled out from within the main housing  113 ; 
         FIG. 12  shows the data storage unit  101  with the support unit  112  pulled out from the main housing  113  and with the lid  505  of the support unit  112  open; 
         FIG. 13  shows a second data storage unit  1301  embodying the present invention; 
         FIG. 14  shows a side view of the data storage unit  1302 ; 
         FIGS. 15A to 15H  show the process of replacing the unit  1302  with a support unit containing disk drives; 
         FIG. 16  shows a data storage unit in which one support unit comprises a different type of disk drive to the other two supports units; 
         FIG. 17A  shows a plan view of the support unit  112  without the fan module  406  and without the disk drives  401  fitted; and 
         FIG. 17B  shows a plan view of the support unit  1602  without the fan module  406  and without the disk drives  401  fitted. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     
       FIG. 1 
     
     A data storage unit  101  embodying the present invention is shown mounted in a rack  102  in  FIG. 1 . 
     The rack  102  is built in accordance with an international standard, EIA-310, and therefore has a specified depth from the front panel  103  to the back panel  104 . Similarly, the rack  102  has a standard sized gap between a first rail  105  and a second rail  106  which form part of the front panel  103  and are provided with holes  107  allowing equipment such as data storage unit  101  to be attached and supported. 
     The data storage unit  101  has a generally rectangular form, but includes a mounting plate  108  at its front end that is provided with apertures allowing the data storage unit to be attached to the rails  105  and  106  of the rack  102 . Most of the data storage unit  101  has a width configured to allow said unit to pass between the rails  105  and  106  of the rack  102 , and a depth that ensures a gap is provided between the rear end  109  of the data storage unit  101  and the back panel  104  of the rack  102  when the storage unit  101  is mounted, as shown in  FIG. 1 . 
     The data storage unit  101  comprises several data storage elements. In the present example, the data storage unit  101  comprises sixty data storage elements in the form of sixty 3.5″ (three point five inch) disk drives arranged in three groups of twenty. Each group of twenty disk drives is mounted on one of three support units  110 ,  111  and  112  located within a space provided within the main part  114  of the data storage unit  101 . Each disk drive is connected via its support unit to a control unit via a flexible cable connecting its support unit and a control unit mounted within the data storage unit  101 . 
     The data storage unit is connected to one or more computers, which access and/or edit data stored on the disk drives. Communication between the data storage unit and the one or more computers may be made by known interfaces and protocols, for example communication may be made over a network such as a LAN, WAN, SAN, the Internet, etc. Similarly, communication may be made over a wire, optical link, radio link, etc. 
     The data storage unit  101  has an outer main housing  113  in which the three support units  110 ,  111  and  112  are independently supported, such that they can each slide forward out of the main housing  113  independently of the other two support units. 
     
       FIGS. 2 and 3 
     
     The data storage unit  101  is shown in each of  FIGS. 2 and 3 .  FIG. 2  provides a view of the front top and left side of the data storage unit  101 , along with rack rails  223 .  FIG. 3  provides a view of the rear, top and left side of the data storage unit  101 . The main housing  113  is formed from several aluminium alloy extrusions and has a substantially rectangular box form with closed walls at the left, right, top and bottom sides and two open ends at the front and rear. The open-ended front allows access into a space formed within the main housing  113  for the three support units  110 ,  111  and  112 . Similarly, the open rear end allows access into the main housing for components located towards the rear of the data storage unit  101 . The support units  110 ,  111  and  112  occupy approximately the front two thirds of the main housing while other components occupy the rear one third. 
     The extrusions forming the main enclosure  113  are also formed with grooves such that a groove  222  extends along the outside surface of each of the left and right side walls  202  and  203 . The grooves  222  are provided for receiving a correspondingly shaped rail that is horizontally mounted within a rack. Suitable rack rails  223  are shown in  FIG. 2 . The rails  223  are bolted within the rack  102  so that they extend horizontally from the front to the back of the rack. The ends of the grooves  222  at the rear end of the data storage unit  101  are then located onto the front of the rails  223  and the data storage unit  101  is then slid back into position along the rails  222 . The data storage unit  101  is then secured in position by screws located through the mounting plate ( 108 , shown in  FIG. 1 ). 
     In the present embodiment the rails  223  are themselves formed from extruded aluminum alloy. 
     Apertures  221  are provided in the front face of each the support units  110 ,  111  and  112  to allow a flow of air through the support unit for the purposes of cooling the data storage elements, the control units  204  and  205 , and the power supplies  206  and  207 . 
     A printed circuit board referred to as the mid-plane  201 , is mounted within the main housing  113  perpendicular to the side walls  202  and  203  of the main housing  113 . The mid-plane  201  (whose position within the outer main housing  113  is indicated in dotted lines) extends between the left and right side walls  202  and  203  of the main housing  113  and provides electrical connections between the other various components of the data storage unit  101 . 
     Towards the rear of the unit  101 , behind the mid-plane  201 , the main housing  113  contains two control units  204  and  205  and power supplies  206  and  207 . The power supplies  206  and  207  are dual redundant power supplies. It is therefore possible for the data storage unit  101  to operate with just one of the power supplies operational. The control units are also redundant, so it is possible for the data storage unit  101  to operate with just one of the control units  206  or  207 . 
     The rear surfaces of the power supplies  206  and  207  are provided with electrical sockets to allow connection to an electricity supply. The power supply units  206  and  207  are each configured to transform an electricity supply and thereby provide a suitable DC electricity supply for the data storage unit  101 . 
     The control units  204  and  205  comprises control circuitry for controlling inputs and outputs to the disk drives, they are provided with suitable sockets  208 ,  209  to provide connection to a network, etc as previously mentioned. 
     The data storage unit  101  also comprises three cable modules  210 ,  211  and  212 . The cable modules are each linearly aligned with an associated one of the support units  110 ,  111  and  112 . Thus, support unit  110  has an associated cable module  210 , support unit  111  has an associated cable module  211 , and support unit  112  has an associated cable module  212 . The three cable modules are all similarly configured and perform similar functions in respect of their respective support unit. The cable modules each define a space in which electric cables connecting the support units  110 ,  111 ,  112  and the control units  204 ,  205  (via the mid-plane  201 ) reside. Each cable module comprises a pair of cables, of which one cable is redundant in normal use. 
     As shown in  FIG. 3 , each of the three cable modules  210 ,  211  and  212  is provided with a handle  351  to allow a user to pull the respective cable module out from the back of the main housing  113 . The cable modules  210 ,  211 ,  212  will be further described below. 
     The front face of each of the three support units  110 ,  111  and  112  is provided with an array of indicator elements. In the present embodiment, the indicator elements comprise LED&#39;s (light emitting diodes)  261 . The LED&#39;s  261  are selectively illuminated in accordance with signals received from the respective support unit to indicate the status of a number of functional elements of the data storage unit. For example, LED&#39;s are illuminated to indicate correct operation of each power supply unit, each control unit, fans located within the support units, connection cables, etc. 
     The mid-plane  201  comprises a circuit board that supports electric connectors  324 ,  325 ,  326 ,  327 ,  340 ,  341  and  342  configured to co-operate with electric connectors on each of the control modules  204  and  205 , the power supplies  206  and  207  and the cable modules  210 ,  211  and  212  respectively. 
     
       FIG. 4 
     
     A simplified cross-section view of the data storage unit  101  is shown in  FIG. 4 . The cross-section is through the support unit  112  of data storage unit  101  in a plane parallel to the side wall  203  of the outer main housing  113  and perpendicular to the mid-plane  201 . 
     In the present example, each of the support units  110 ,  111  and  112  is similarly configured. Thus, it will be understood that the following description relating to support unit  112  also applies to support units  110  and  111 . 
     The support unit  112  contains twenty 3.5″ (three point five inch) disk drives  401  mounted between support members  402  on a printed circuit board  403 , which will be referred to as the support unit board  403 . The disk drives are arranged in five rows, such that each row has four disk drives. Each disk drive  401  is electrically connected with circuitry on the support unit board  403  by electric connectors  404  mounted on the support unit board. 
     The support unit board  403  also has a pair of connectors  405  (one of which may be seen in  FIG. 4 ) for providing electric power to a first fan module  406 , via a riser card  407 . The riser card  407  is a relatively small printed circuit board having a lower end section shaped to be located within, and to provide electrical connection with, the connector  405 . The riser card  407  itself has a female electric connector  408  fixed to its upper end. The connector  408  receives a lower portion of a further printed circuit board  409 , that forms a part of the fan module  406 , and thereby provides power to the fan module  406 . 
     The support unit board  403  has a further connector  410 , close to its front end, for providing electric power to a second fan module  411  that forms the front of the support unit  112 . The fan module  411  is provided with two pairs of hooks  451  and each side wall of the support unit  112  is provided with a pair of inwardly extending pins  452 . The fan module  411  is thus suspended on the pins  452  by the hooks  451 . A screw  453  located through a hole in each side wall of the support unit  112  is used to lock the fan module  411  in place. 
     During operation, the first fan module  406  and the second fan module  411  are each configured to provide a flow of air through the support unit  112  and through the control units (such as control unit  305 ) and power supply units (such as power supply unit  307 ). In the present embodiment, the fan modules  406  and  411  are each capable of providing a sufficient flow of air to cool the disk drives  401 , independent of the operation of the other fan module. Thus, in the event that one fan module fails, the other fan module is able to provide the necessary airflow until the defunct fan module is replaced. 
     As mentioned previously, the cable module  212  contains a pair of electric cables. One of the pair of electric cables, cable  412 , is shown in  FIG. 4 . Each of the electric cables terminates at one end with a connector  413 , and terminates at the other end with a connector  414 . The connectors  413  connect with a respective one of the connectors  341  on the mid-plane  201 , while the connectors  414  connect with a corresponding one of a pair of electric connectors  415  mounted at the rear edge of the support unit board  403 . 
     The cable  412  is of sufficient length to allow the support unit  112  to slide forwards out from its main housing  113 , while the cable is still connected to the mid-plane  201 . Consequently, when the support unit  112  is in its normal operating position, shown in  FIG. 4 , a portion of the cable  412  is bent into several loops within the cable module  212 . A guide module  416  is mounted within the cable module  212 , and as will be described below, this module ensures that no such loops are formed in the cable  412  outside of the cable module. 
     
       FIG. 5 
     
     A further cross-section view of the data storage unit  101  is shown in  FIG. 5 . The cross-section is through the support units  110 ,  111  and  112  of data storage unit  101  in a plane perpendicular to the side wall  203  of the outer main housing  113  and parallel to the mid-plane  201 . 
     The support unit board  403  is supported above a floor  501  of the support unit  112 . The floor  501  is formed as part of a U-shaped channel  502 , having side walls  503  and  504 . A lid  505  covers the open end of the U-shaped channel  502  so that the disk drives  401  effectively reside within a rectangular tube through which the fan modules  406  and  411  blow a stream of air during operation. 
     The lid  505  is attached to one side wall  504  of the U-shaped channel by a hinge mechanism  506 . In the present embodiment the U-shaped channel and the lid are formed as aluminium extrusions, and the hinge mechanism  506  is formed as part of the extrusions. An upper edge of the side wall  504  is formed with an outer surface having a partial cylindrical shape. The corresponding edge  507  of the lid is formed with a slot having an inner surface that has a partial cylindrical shape of similar diameter, such that said inner surface of the slot is located around said outer surface of the upper edge of the side wall  504 . 
     When the support unit  112  is manufactured one end of the upper edge of the side wall  504  is located within one end of the cylindrically shaped slot of the lid  505  and then the lid is slid into position. The lid is provided with a transverse notch (not shown) on its edge  507 , and a locking pin (not shown) is located in the side wall  504  at the location of the notch. Consequently, the locking pin acts on the sides of the notch to prevent the lid  505  from sliding out of position along the side wall  504 . 
     The main housing  113  is provided with a pair of slots on the inside of its upper wall  514  and a similar pair of slots on the inside of its lower wall  515 . These slots are configured to receive respective upper and lower edges of partition wall elements  516  and  517 , which partition the front of the main housing into three compartments. Each of the compartments is configured to receive one of the support units  110 ,  111  or  112 . In the present embodiment, partition wall elements  516  and  517  are also formed as aluminium extrusions. 
     The support units  110 ,  111  and  112  are each supported within the main housing  113  by a pair of slide rails. Support unit  110  is supported by slide rails  508 A and  508 B, support unit  111  is supported by slide rails  509 A and  509 B and support unit  112  is supported by slide rails  510 A and  510 B. In the present embodiment the slide rails are friction slides in which an inner metal rail  512  is held within an outer metal rail  511  by sections  513  of a low friction plastics material. 
     The main housing  113  is formed with recesses along the inside of its side walls  202  and  203  that are shaped to receive the outer rails  511  of slide rails  508 A and  510 B. Similarly, the partition wall elements  516  and  517  are formed with recesses shaped to receive the outer rails  511  of slide rails  508 B and  509 A, and  509 B and  510 A respectively. As shown in  FIG. 5 , the two slide rails of each support unit  110 ,  111  and  112  are positioned at different heights, such that the top edge of the lowest slide rail is located at a height that is below the bottom edge of the highest slide rail. Consequently, the two slide rails  508 B and  509 A supported by the partition wall element  516  are also at different heights, which means that the width of space taken up by the partition wall and the slide rails is minimised. (The situation is also similar for slide rails  509 B and  510 A supported by partition wall element  517 .) Thus by vertically staggering the slide rails in this manner, it is possible for the data storage unit  101  to benefit from the use of the slide rails and contain the three support units  110 ,  111  and  112 , while still being sufficiently narrow to fit within a 19″ rack. 
     It may be noted that the side walls  202  and  203  of the main enclosure  113  are provided with recesses along their inner faces which are dimension to receive the slide rails  508  and  511 . As may be seen from  FIG. 5 , the recesses receiving the slide rails  508  and  511  are provided at different heights so that they correspond with difference in heights of the slide rails on the support units. As mentioned above, the main housing  113  is formed from several aluminium alloy extrusions, and the recesses receiving the slide rails  508  and  511  are formed during the extrusion process. 
     The rack rails  223 , previously shown in  FIG. 2 , are shown in dotted outline in  FIG. 5  located in their respective grooves  222 . In the present embodiment the rack rails  223  have a T-shaped cross-section and the grooves have a corresponding T-shaped cross-section. Thus the sides of the grooves  222  are provided with a pair of inwardly extending lips  522  that locate behind outwardly extending lips  523  on the rails  223 . 
     In an alternative embodiment, the rails  223  and grooves  222  are provided with an L-shaped cross-section, and consequently each groove has a single lip for locating behind the single lip formed along the rail. 
     As will be apparent from  FIG. 5 , most of the width of the rails  223  extends into the side walls of the main enclosure  113 , and this is made possible by the presence of the grooves  222  formed in the side walls of the main enclosure  113 . As a result, the data storage unit  101  is simply and securely mounted within the rack ( 102 ) without unnecessarily reducing the width available for accommodating disk drives. 
     In an alternative embodiment, the U-shaped channel  502  is made from sheet steel formed into the U shape by bending. 
     In an alternative embodiment, the slide rails  508 A,  508 B,  509 A,  509 B,  510 A and  510 B are each replaced by three section friction slides and have metal to metal sliding surfaces. 
     
       FIG. 6 
     
     A perspective view of the rear of the support unit  112  removed from the outer main housing  113  is shown in  FIG. 6 , with the fan module  406  removed. 
     As shown in  FIG. 6 , the lid  505  encloses most of the top of the U-shaped channel  502  formed by the floor  501  and side walls  503  and  504 . However, the lid  505  does not extend over a portion of the support unit  112  that normally contains the fan module  406 . Consequently, the fan module  406  may be removed from the U-shaped channel  502  without opening the lid  505 . 
     The fan module  406  comprises a pair of fans  601  and  602  mounted on a frame  603  formed of a plastics material. Each side wall of the U-shaped channel is provided with a pair of slots  604 , and each side of the frame  603  is provided with a pair of outwardly extending members  605  configured to fit into the slots  604 . Thus, when fitted, the outwardly extending members  605  located in the slots  604  suspend the fan module  406 . 
     The fans  601  and  602  are electrically connected to the pair of downward extending printed circuit boards  409 . The riser cards  407  (one of which may be seen in  FIG. 6 ) are symmetrically arranged within the U-shaped channel such that, when the outwardly extending members  605  are located in the slots  604 , the lower portion of the printed circuit boards  409  are located within the electric connectors  408  on the riser cards  407 . This arrangement allows a fan module  406  to be replaced simply by unplugging the fan module, and plugging in a new module. 
     
       FIG. 7 
     
     The cable module  212  is shown in  FIG. 7 , removed from the data storage unit  101 . The cable modules  210 ,  211  and  212  are substantially identical, and therefore it will be understood that the following description of cable module  212  similarly applies to the other two cable modules. The cable module  212  is formed on a metal U-shaped channel having side walls  701  and  702  extending upward from a floor. The rear end of the cable module  212  is closed by a rear wall  703 , and the front end of the unit  212  is partially closed by a front wall  704  that supports two similar electric connectors  413 . The electric connectors  413  are rigidly fixed to the front wall  704  of the cable module  212  and are configured to mate with the corresponding connectors  342  on the mid-plane  201 . The electric connectors  413  are each electrically connected to a respective electric cable  412 . The electric cables  412  are therefore connected at a first end to a respective one of the electric connectors  413  and are terminated at their opposite end by a respective one of the electric connectors  414 . The electric connectors  414  are configured to mate with a corresponding one of the electric connectors  415  on the support unit  112 . 
     The cable module  212  also comprises a partition wall  713  which extends centrally along the cable module  212  from the rear wall  703  to the front wall  704 , between the two electric cables  412 . The partition wall  713  effectively splits the space within the cable module  212  in two, and ensures that the two electric cables  412  do not mechanically interfere with each other. 
     A narrow top wall  721  extends forwards from the uppermost edge of the rear wall  701 , and a safety device  722  is mounted on the top wall  721 . The safety device  722  comprises a button  723  that extends up through a correspondingly formed aperture  724  in the top wall  721 . The button  723  is mounted on a spring such that it is urged upwards to a position in which it protrudes above the top wall  721 , but under finger pressure it may be pushed down to be level with the top wall. Consequently, before positioning the cable module  212  in its operational position in the data storage unit  101 , the button  723  must be depressed 
     Unlike conventional signal cables, each of the cables  412  comprises both power wires and high speed signal wires (SAS (serial attached SCSI (Small Computer System Interface) wires) in a single cable. 
     
       FIG. 8 
     
     The cable module  212  is shown again in  FIG. 8 , but with the electric cables  412  in dotted outline only.  FIG. 8  also shows a rear view of the guide module  416  that is fixed within the cable module  212 . The guide module  416  comprises a housing  802  formed of a solid plastics material. The housing  802  is held rigidly in place by screws  803 , which extend through the side walls  701  and  702  of the cable module  212 . The housing  802  supports inner components of the guide module  416 , as will be described below, and also provides a barrier between the electric cables  412  and these inner components. 
     The spring  810  on which the button  723  is mounted is shown in dotted outline in  FIG. 8 . In the present embodiment, the spring  810  and the button  723  are formed of a single strip of bent metal that is attached at one end to the underside of the top wall  721  by a pair of screws  811 . The other end of the strip is bent to form the button  723  that protrudes up through the aperture  724 . 
     
       FIG. 9 
     
     The cable module  212  is shown again in the side view of  FIG. 9 . 
     The side walls  701  and  702  extend along the length of the floor panel  901  but have an upper portion  902  that extends substantially further forward than the floor panel  901 . Thus, an aperture is defined between the side walls  701  and  702 , the front wall  704  and the floor panel  901 . The guide module  416  (shown in dotted outline in  FIG. 9 ) is mounted within the cable module  212  adjacent to this aperture. 
     The guide module  416  comprises a pair of constant force springs  903  axially mounted on a shaft  904  and having a free end attached to a connecting rod  905 . The constant force springs are shown in  FIG. 9  in their stable, coiled up, configuration, having only a short substantially straight end portion connected to the connecting rod  905 . However, it will be understood that by pulling on the connecting rod  905  it is possible to uncoil the constant force springs  903  such that they extend substantially linearly forward from the front of the cable module  212 . 
     The constant force springs  903  each have a shape similar to that of a retractable steel tape measure which may be coiled up into a housing but when extended forms a substantially rigid straight length of material that resists bending. It is this feature of the constant force springs  903  that ensures that the cables  412  do not form bends out side of the cable module  212  when they are pushed back into the cable module after being extended. 
     The connecting rod  905  fixed to the free end of the constant force springs  903  is retained within a connection module  910  which also contains the electric connectors  414  at the ends of the cable  412 . Thus, when the connectors  414  are pulled forward from the cable module  212 , the constant force springs are uncoiled and extend forward, above the portions of the electric cables  412  that extend from the cable module. 
     In an alternative embodiment, the floor panel  901  is reduced in length such that it only extends by approximately 75 mm from the rear wall  703 . Consequently, any potential problems with the electric cables  412  binding on the front edge of the floor panel are avoided. 
       FIGS. 10A ,  10 B and  10 C 
     The connection module  910  is shown in further detail in the perspective view of  FIG. 10 , the simplified bottom view of  FIG. 10B  and simplified cross-sectional view of  FIG. 10C . 
     The connection module  910  comprises a saddle element  1001  in which the electric connectors  414  are mechanically retained. An end plate,  1002 A and  1002 B respectively, is fixed to the saddle element  1001 , at either side of the connectors  414 . The end plates  1002 A and  1002 B each define a notch  1003  having a width and shape configured to receive and retain the connecting rod  905  that is fixed to the end of the constant force springs  903 . 
     As illustrated in  FIGS. 10B and 10C , the saddle element  1001  has a hole  1004  extending upward from its lower surface  1005 . The hole  1004  extends up to an enlarged void  1006 . This feature may be used to latch the connection module in a temporary, convenient position during some repair or updating procedures that may be performed on the data management unit  101 , as will be further discussed below. 
     In an alternative embodiment, the end plates  1002 A and  1002 B each define a hole rather than a notch. The holes are dimensioned to receive respective ends of the connecting rod  905 . The rod is located within the holes while the end plates are loosely attached and then screws  1007  holding the end plates in place are tightened. 
     
       FIG. 11 
     
     For a number of repair and maintenance procedures it is necessary to slide a selected one of the support units ( 110 ,  111  or  112 ) out from within the outer main housing  113  of the data storage unit. For many such procedures the data storage unit  101  may remain in its operational condition, thereby minimising inconvenience, minimising loss of business, etc. 
     The data storage unit  101  is shown in  FIG. 11 , mounted in the rack  102  with support unit  112  pulled out from within the main housing  113 . The data storage unit is operating, and therefore, provided they are functioning properly, the fan modules  411  and  406  continue to provide a flow of cooling air through the tube formed by the U-shaped channel  502  and the lid  505 . 
     In the event that the fan module  406  has stopped working properly, this may be unplugged and replaced with a new module, with the data storage unit  101  still operating and with the lid  505  closed. 
     If the fan module  411  is faulty, the screws  453  are removed, the lid  505  is opened and the fan module is unhooked from the pins  451 . The fan module  411  may then be replaced with a new module. 
     
       FIG. 12 
     
     The data storage unit  101  is shown in  FIG. 12  with the support unit  112  pulled out from the main housing  113  and with the lid  505  of the support unit  112  open. Thus, the disk drives  401  within the support unit  212  are accessible. A faulty disk drive has been removed from the support unit  112  leaving an empty bay  1201 . A new disk drive  1202  is being inserted into the empty bay  1201 , to replace the faulty disk drive. While the lid  505  is open, a portion of the desired airflow is lost though the top of the U-shaped channel  502 . However, as soon as the disk drive  1202  is in position, the lid may be closed to re-establish the desired airflow. 
     
       FIG. 13 
     
     A second data storage unit  1301  embodying the present invention is shown in  FIG. 13 . The data storage unit  1301  is identical to data storage unit  101  except that the middle unit  1302  is a different type to units  1310  and  1312 . Units  1310  and  1312  are essentially the same as support units  110  and  112  of data storage unit  101 , and each contain twenty disk drives. However, unit  1311  is contains no disk drives, and is not configured to contain any disk drives. Consequently the overall cost of the data storage unit  1301  is low compared to that of data storage unit  101 . 
     As will be further described below, the unit  1302  is provided with fan modules  406  and  411 , such that it is able to assist units  1310  and  1312  with air cooling the control units and power supply units of the data storage unit  1301 . The unit  1302  is also provided with LED&#39;s  1361  such that it is able to provide indications relating to status in a similar manner to the LED&#39;s  261  of the support units  1310  and  1312 . 
     
       FIG. 14 
     
     A side view of the data storage unit  1302  is shown in  FIG. 14 . Like support unit  112 , the unit  1302  comprises a U-shaped channel  1401  enclosed at the top by a lid  1402 . The U-shaped channel  1401  and lid  1402  are formed from extruded aluminium, in the same way as U-shaped channel  502  and lid  505 , but the U-shaped channel  1401  and lid  1402  are shorter in length, being only approximately twenty centimeters long. 
     Inner slide rail  1403 A and  1403 B are fixed to the side walls of the U-shaped channel  1401  in a similar configuration to the inner rails mounted on support unit  112 . Thus, the unit  1302  is mountable within a data storage unit, such as data storage unit  101  or  1301  in place of a support unit such as support unit  112 . 
     A printed circuit board  1405  is mounted within the U-shaped channel parallel with the floor of said channel. The printed circuit board  1405  has an electric connector  1406  close to its front edge for supplying electric power to a fan module  411 , and a pair of connectors  1407  for providing electric power to a fan module  406  via riser cards  407 . A pair of connectors  1408  are mounted on the rear edge of the printed circuit board that are configured to connect to the usual cables  412  of a cable module, such as cable module  212 . 
     The printed circuit board  1405  comprises no circuitry, connectors or components relating to disk drives, because it is not intended that it should ever contain disk drives. Consequently, the unit  1302  is considerably less expensive to manufacture than a support unit such as support unit  112 . 
       FIGS. 15A ,  15 B,  15 C,  15 D,  15 E,  15 F,  15 G,  15 H 
     The unit  1302  has similar types of slide rail and electrical connections to the support units, such as support unit  1310  and  1312 . Consequently, it is possible to remove the unit  1302  from the data storage unit  1301  and replace it with a support unit of the same type as support units  1310  and  1312 . Thus, the data storage unit  1301  may be upgraded from a data storage unit comprising forty disk drives to a data storage unit having sixty disk drives. 
     Similarly, it is also possible to have a data storage unit containing one support unit, such as unit  1301 , having twenty disk drives along with two units, such as unit  1302 , which contain no disk drives, so that the data storage unit only has twenty disk drives in total. This data storage unit may then be upgraded to contain forty or sixty disk drives by replacing one or both of the units, like unit  1302 , that contain no disk drives, with support units, like units  1310 ,  1311 ,  1312 , having twenty disk drives. 
     The process of replacing the unit  1302  with a support unit containing disk drives is illustrated by  FIGS. 15A to 15H . In each of these figures, the data storage unit  1301  is shown in cross-section, the cross-section being through the middle cable module  211  (of the three cable modules) and parallel to the side walls of the data storage unit  1302 . 
     The data storage unit  1301  is shown in its initial state in  FIG. 15A . The data storage unit  1301  is operating and the fan modules  406  and  411  of unit  1302  are operating to provide a flow of air for cooling the control units, such as  204 , and power supply units, such as  206 . The unit  1302  is to be replaced while the rest of the data storage unit  1301  continues to operate. Consequently, in the configuration of  FIG. 15A , the cable  412  is providing electric power to the unit  1302 . (Typically the power supplied to the units, including unit  1302  is 480 VA.) 
     In order to isolate the unit  1302  from the electric power supply, the cable module  211  is pulled out from its operational position within the data storage unit  1301 , until the button  723  pops up under the action of its spring. This movement is sufficient to disengage the electric connectors  413  of the cable module  211  from the corresponding connectors  341  on the mid-plane  201 . Thus, the power to the unit  1302  is cut. 
     It may be noted that the person pulling the cable module  211  from its operating position does not have to go near the connector  413  at the front of the cable module to pull it from the connector on the mid-plane  201 . In fact, the rear wall  703  of the cable module  211  provides a safety barrier preventing access to the connectors and the mid-plane  201 . 
     The data storage unit  211  is shown in  FIG. 15B  after this movement of the cable module  211 . Because the button  723  is protruding up, it prevents the cable module  211  from being accidentally re-inserted into its operating position. Therefore, the unit  1302  may now be safely worked on. 
     The unit  1302  is then slid out from within the main housing  113  of the data storage unit  1301  as shown in  FIG. 15C . The U-shaped channel  1401  is completely outside of the main housing  113  but the unit  1302  is still suspended from the data storage unit  1301  by its slide rails  1403 A and  1403 B. 
     A retaining tool  1505  is then attached to the front lower edge of the main housing  113  in a position between the two cables  412  of the cable module  211 . The retaining tool  1505  is shown attached to the main housing in  FIG. 15D . 
     The retaining tool  1505  comprises a flat plate  1506  having a hook-shaped end  1507  extending to one side of the flat plat. The hook-shaped end  1507  is configured to hook around the front edge of the main housing  113 . A post  1508  extends from the other side of the flat plate  1506 . The post  1508  has an enlarged head configured to fit through the hole  1004  formed in the under-side of the connection module  910 . 
     After positioning the retaining tool  1505 , the electric connectors at the ends of the electric cables  412  are disconnected from the electric connectors  1408  of the unit  1302 . The connection module  910 , which carries the electric connectors at the end of the cables  412 , is then temporarily stored by locating the hole  1004  on its under-side over the post  1508 . The enlarged head of the post  1508  enters the void  1006  within the connection module  910  so that the connection module  910  is effectively hooked onto the post  1508 . 
     The data storage unit  1301  is shown in  FIG. 15E  with the connection module  910  temporarily stored on the retaining tool  1505 . 
     It may be noted that the connecting rod  905  fixed to the end of the constant force spring  903  remains located within the notches  1003  of the connection module  910 . Consequently, the connection module  910  is urged inwards into the main housing  113 , but is held in place by the retaining tool  1505 . 
     The unit  1302  is then slid further out from the data storage unit  1301  and its inner slide rails  1403 A and  1403 B disconnected from the outer rails  511  of the main housing data storage unit  1301 . 
     A new supporting unit ( 1510  in  FIG. 15  F) is then fitted in place of unit  1302 . 
     In the present example, the new support unit  1511  is identical to support unit  112  and its features will be given the same reference numerals as those of support unit  112 . 
     Firstly, the inner rails  512  of the new supporting unit  1510  are located in the outer rails  511  within the data storage unit  1301 . The new supporting unit  1510  is then slid toward the main housing  113  to a position in which its electric connectors  415  at the rear edge of its support unit board  403  are accessible. The connection module  910  is then unhooked from the retaining tool  1505  and the electric connectors carried by the connection module  910  are connected to the electric connectors  415  of the support unit  1510 . The retaining tool is then removed. 
     The data storage unit  1301  with the new support unit  1510  connected is shown in  FIG. 15F . 
     The support unit  1510  is then slid into the main housing  113  into it normal operating position as shown in  FIG. 15G . 
     Finally, the button  723  on the cable module  211  is depressed and the cable module  211  is pushed back into its normal operating position, such that the connectors  413  of the cable module  211  reconnect with the connectors  341  on the mid-plane  201 . Thus, power is supplied to the support unit  1510  and the control units  204  are able to access the disk drives  401  of the support unit  1510 . 
     
       FIG. 16 
     
     In a data storage device embodying a further aspect of the present invention, at least one support unit comprises a different type of disk drive to the other two supports units. Such a data storage device  1601  is shown in cross-section in  FIG. 16 . The data storage device contains two support units  1611  and  1612  that are substantially identical to support unit  112 , and so each contain twenty 3.5″ hard disk drives  401 . The third support unit  1602  is similarly configured to support units  1611  and  1612  but differs in that it contains a support unit board  1603  having connectors  1604  arranged to connect to thirty two 2.5″ hard disk drives  1605 . The hard disk drives  1605  have the standard 2.5″ hard disk drive form factor and a thickness of 15 mm. 
     The support unit  1602  has similar slide rails to the other support units and similar electric connectors for connecting to the main part of the data storage unit. Consequently, the support unit  1602  may be fitted as a replacement for a support unit containing 3.5″ disk drives or a unit of the same type as unit  1302 . When making such a replacement, it may be performed using a similar method to that described with reference to  FIGS. 15A to 15H . 
     
       FIGS. 17A and 17B 
     
     A plan view of the support unit  112  without the fan module  406  and without the disk drives  401  fitted is shown in  FIG. 17A , and a plan view of the support unit  1602  without the fan module  406  and without the disk drives  401  fitted is shown in  FIG. 17B . 
     The support unit card  403  of the support unit  112  is a printed circuit board on which are mounted twenty connectors for connecting to a respective twenty 3.5″ disk drives. The support unit card  403  also has: the two electric connectors  405  for receiving the riser cards  408 ; the connector  410  for connecting to the fan module  411 ; and two electric connectors  415  for connecting to the cables of a cable module, such as cable  412  of cable module  212 . 
     The support unit  1602  has a similar form to support unit  112  and contains the support unit card  1603  having similar dimensions to the support unit card  403  and similar connectors  410 ,  405  and  415  as described for support unit card  403 . However, support unit card  1603  is a printed circuit board on which are mounted the 32 connectors  1604  arranged to receive 2.5″ disk drives. The connectors are arranged in seven rows, with each of six rows containing five connectors and one row containing two connectors. 
     In each of the described embodiments, the support units of a data storage unit (such as data storage unit  101 ) are individually mounted on slide rails and connection from the support units to the mid-plane of the data storage unit is made by flexible cables. As a consequence, there is relatively little mechanical vibration transmitted from the support unit card of one support unit (such as card  403  of support unit  112 ) to the support unit card of another support unit. This is particularly true of the above-described embodiments in which the support units are formed of aluminium, because the aluminium tends to be a poor conductor of the vibrations that are produced. 
     A further alternative embodiment makes use of the vibrational isolation of the support units. In this embodiment, a data storage unit has SAS disk drives in one support unit and SATA disk drives in another of the support units. In general, SAS disk drives generate more vibrational energy than SATA disk drives, and the SAS disk drives are also capable of operating in conditions where higher levels of vibrational energy exists. Consequently, it is possible to operate a number of disk drives in close proximity to each other provided they are of the same one of these two types, but operating SAS disk drives and SATA disk drives in close proximity can adversely effect the operational efficiency of the SATA disk drives. However, in the present alternative embodiment, the SATA disk drives are located in a separate support unit to the SAS disk drives, and so only a small proportion of the vibrational energy generated by the SAS disk drives is conducted to the SATA disk drives. As a result, the two disk types are able to reside successfully in a single data storage unit.