Patent Publication Number: US-2015072540-A1

Title: Apparatus for housing plug-in unit and plug-in unit

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-188580, filed on Sep. 11, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiment discussed herein is related to an apparatus for housing a plug-in unit and a plug-in unit. 
     BACKGROUND 
     Advancement in information apparatuses has achieved the practical use of apparatuses having a plug-in structure in which numerous packages and units with electronic circuits and storage devices are housed in shelves or slots in an insertable and removable manner. In an apparatus including a plug-in structure that is capable of housing units in a plurality of slots, a connector that exchanges electrical signals with the unit is provided in the back of each slot. When the unit is inserted into the slot, the unit is connected by a connector on the unit being plugged into the connector in the back of the slot. Such a unit is replaceable, and is also referred to as a replaceable unit or a plug-in unit. 
     Meanwhile, when the packages and units (referred to, hereinafter, as plug-in units) that are able to be inserted into and removed from the apparatus having a plug-in structure are of a single type, any plug-in unit may be attached to any slot. However, there are some apparatuses having a plug-in structure that allow attachment of a plurality of types of plug-in units with different functions. 
     Japanese Laid-open Patent Publication No. 8-316667 discloses a package-misinsertion prevention mechanism for enabling plug-in units that respectively correspond with the slots to be inserted without error in an apparatus having a plug-in structure that supports such a plurality of types of plug-in units. The apparatus disclosed in Japanese Laid-open Patent Publication No. 8-316667 is provided with a separate connector in a position in front of a normal insertion position. The separate connector is initially coupled with a package. During normal insertion, the separate connector sends a current to an electromagnet within the package and attracts a magnet in a movable piece, thereby drawing the movable piece into the package. Therefore, the package is directly inserted and coupled. On the other hand, during misinsertion, the separate connector does not send a current to the electromagnet within the package, and the movable piece remains projecting from the package. Therefore, the movable piece comes into contact with an insertion blocking section and insertion of the package is blocked. 
     In addition, to prevent a plug-in unit from being incorrectly inserted, there is also a unit-misinsertion prevention mechanism in which an obstruction, such as a pin, is provided in the slot of the apparatus having a plug-in structure or on a side surface of the plug-in unit. An example of this misinsertion prevention mechanism is illustrated in  FIGS. 1A to 1D . As illustrated in  FIG. 1A , a pin  92  is provided in a projecting manner in the innermost portion of a slot  91  in an apparatus having a plug-in structure  90 . The pin  92  is attached at the factory in which the apparatus having a plug-in structure  90  is manufactured. As illustrated in  FIG. 1C , the attachment position of the pin  92  differs depending on the type of plug-in unit  93  to be inserted into the slot  91 . On the other hand, a key groove  94  is formed in an end portion of the plug-in unit  93  to be inserted into the slot  91 . The setting position of the key groove  94  also differs depending on the type of plug-in unit  93 . 
       FIG. 1A  illustrates a state in which a correct plug-in unit  93  is inserted into a certain slot  91 . In this instance, the position of the pin  92  and the position of the key groove  94  match. Therefore, in this instance, as illustrated in  FIG. 1B , when the plug-in unit  93  is inserted into the slot  91 , the pin  92  enters the key groove  94 . As a result, the plug-in unit  93  is completely inserted into the slot  91 .  FIG. 1C  illustrates a state in which an incorrect plug-in unit  93  is inserted into another slot  91 A. In this instance, the position of the pin  92 A and the position of the key groove  94  do not match. Therefore, in this instance, as illustrated in  FIG. 1D , when the plug-in unit  93  is inserted into the slot  91 A, the tip of the pin  92  comes into contact with the end surface of the plug-in unit  93  on which the key groove  94  is not provided. Therefore, the plug-in unit  93  is not inserted into the slot  91 . 
     SUMMARY 
     According to an aspect of the invention, an apparatus for housing a plug-in unit that includes at least one movable piece including a permanent magnet in a portion of a casing, the apparatus for housing a plug-in unit includes a plurality of slots, each of the plurality of slots houses the plug-in unit; a connector provided in an inner portion of the slot and electrically connects with the plug-in unit; an advancement blocking member disposed within the slot and blocks connection of the plug-in unit to the connector by coming into contact with the movable piece; an electromagnet that generates a magnetic field applied to the permanent magnet of the plug-in unit; and a control device that controls the electromagnet so as to generate a magnetic field having a predetermined polarity that moves the movable piece of the plug-in unit to a position in which the movable piece does not come into contact with the advancement blocking member. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a cross-sectional view of the structures of a slot in an apparatus having a plug-in structure and a plug-in unit that is compatible with the slot in a comparison structure,  FIG. 1B  is a cross-sectional view of a state in which the plug-in unit is mounted in the slot illustrated in  FIG. 1A ,  FIG. 1C  is a cross-sectional view of the structures of a slot in an apparatus having a plug-in structure and a plug-in unit that is not compatible with the slot in a comparison structure, and  FIG. 1D  is a cross-sectional view of a state in which the plug-in unit is not able to be mounted in the slot illustrated in  FIG. 1C ; 
         FIG. 2A  is an explanatory diagram of a disk array and a hard disk device to be housed in the disk array of the present application,  FIG. 2B  is a perspective view of a first example of a misinsertion prevention mechanism that is mounted in the hard disk device illustrated in  FIG. 2A , and  FIG. 2C  is a side view of the misinsertion prevention mechanism of the first example illustrated in  FIG. 2B ; 
         FIG. 3A  is a side view of a misinsertion prevention mechanism in a first variation example of the first example illustrated in  FIG. 2C , and  FIG. 3B  is a side view of a misinsertion prevention mechanism in a second variation example of the first example illustrated in  FIG. 2C ; 
         FIG. 4A  is a perspective view of an internal structure of a slot in the disk array illustrated in  FIG. 2A  that houses the hard disk device illustrated in  FIGS. 2B and 2C , and  FIG. 4B  is a side view of the slot illustrated in  FIG. 4A ; 
         FIG. 5A  is a side view of the slot illustrated in  FIG. 4B  and the hard disk device including the misinsertion prevention mechanism of the first example, 
         FIG. 5B  is a side view of a state in which the hard disk device illustrated in  FIG. 5A  is inserted into the slot, and  FIG. 5C  is a side view of an operation of the misinsertion prevention mechanism when the hard disk device illustrated in  FIG. 5A  is inserted into a slot with which the hard disk does not correspond; 
         FIG. 6A  is a side view of an operation of the misinsertion prevention mechanism when the hard disk device that is inserted into a slot corresponds with the slot, and  FIG. 6B  is a side view of a coupling operation of connectors when the hard disk device that is inserted into a slot corresponds with the slot; 
         FIG. 7A  is an explanatory diagram for explaining a process in which the misinsertion prevention mechanism of the first example operates by electromagnets within a slot when the hard disk device that is inserted into the slot corresponds with the slot, and  FIG. 7B  is an explanatory diagram for explaining a process in which the misinsertion prevention mechanism of the first variation example of the first example operates by electromagnets within a slot when the hard disk device that is inserted into the slot corresponds with the slot; 
         FIG. 8A  is a partial side view of a configuration of a misinsertion prevention mechanism of a second example of the present application that is provided in a hard disk device and a configuration of a corresponding slot,  FIG. 8B  is a partial side view of an operation of the misinsertion prevention mechanism of the second example when a hard disk device that does not correspond with the slot illustrated in  FIG. 8A  is inserted,  FIG. 8C  is a partial side view of an operation of the misinsertion prevention mechanism of the second example when a hard disk device that corresponds with the slot illustrated in  FIG. 8A  is inserted, and  FIG. 8D  is a partial side view of the final position of the hard disk device relative to the slot when the hard disk device that is inserted into the slot corresponds with the slot; 
         FIG. 9A  is a partial side view of a configuration of a misinsertion prevention mechanism of a third example of the present application that is provided in a hard disk device and a configuration of a corresponding slot,  FIG. 9B  is a partial side view of an operation of the misinsertion prevention mechanism of the third example and the final position of the hard disk device relative to the slot when the hard disk device that is inserted into the slot corresponds with the slot, and  FIG. 9C  is a partial side view of an operation of the misinsertion prevention mechanism of the third example when the hard disk device illustrated in  FIG. 9A  does not correspond with the slot; 
         FIG. 10A  is a perspective view of an outer appearance of a hard disk device that is housed in a disk array,  FIG. 10B  is a cross-sectional view taken along a virtual line XB when two misinsertion prevention mechanisms are mounted in the hard disk device illustrated in  FIG. 10A ,  FIG. 10C  is a cross-sectional view taken along a virtual line XC when three misinsertion prevention mechanisms are mounted in the hard disk device illustrated in  FIG. 10A ,  FIG. 10D  is a cross-sectional view taken along a virtual line XD when four misinsertion prevention mechanisms are mounted in the hard disk device illustrated in  FIG. 10A ,  FIG. 10E  is a table indicating the combinations of the polarities of electromagnets and permanent magnets when two misinsertion prevention mechanisms are provided, and  FIG. 10F  is a table indicating the number of misinsertion prevention mechanisms to be mounted and the number of combinations of polarity patterns; 
         FIG. 11A  is a side view of a configuration of a misinsertion prevention mechanism of a fourth example of the present application that is provided in a replaceable unit and a configuration of a corresponding slot, and  FIG. 11B  is a partial side view of an operation of the misinsertion prevention mechanism of the fourth example when the replaceable unit illustrated in  FIG. 11A  does not correspond with the polarity of the electromagnets; 
         FIG. 12A  is a partial side view of another operation of the misinsertion prevention mechanism of the fourth example when the replaceable unit illustrated in  FIG. 11A  does not correspond with the polarity of the electromagnets,  FIG. 12B  is a partial side view of an operation of the misinsertion prevention mechanism of the fourth example when the replaceable unit illustrated in  FIG. 11A  corresponds with the polarity of the electromagnets, and  FIG. 12C  is a partial side view of an operation of the misinsertion prevention mechanism of the fourth example and the final position of the replaceable unit relative to the slot when the replaceable unit illustrated in  FIG. 11A  corresponds with the polarity of the electromagnets; 
         FIG. 13A  is a side view of an example in which an insertion detecting device for a hard disk device is provided in the slot illustrated in  FIG. 4B  of the present application,  FIG. 13B  is an enlarged partial side view of a state in which the hard disk device approaches the insertion detecting device illustrated in  FIG. 13A , and  FIG. 13C  is an enlarged partial side view of a state in which the hard disk device is further inserted into the slot from the state illustrated in  FIG. 13B  and the insertion detecting device is in operation; 
         FIG. 14  is a block circuit diagram of an overall configuration of an apparatus for housing a plug-in unit of the present application in which the insertion detecting device is not provided in the slot; 
         FIG. 15  is a block circuit diagram of an overall configuration of the an apparatus for housing a plug-in unit of the present application in which the insertion detecting device is provided in the slot; 
         FIG. 16  is a flowchart for explaining the procedure in a factory initial setting process that is performed at a factory of the apparatus for housing a plug-in unit of the present application in which the insertion detecting device is not provided in the slot; 
         FIG. 17  is a flowchart for explaining the procedure in the factory initial setting process performed at the factory of the apparatus for housing a plug-in unit of the present application in which the insertion detecting device is provided in the slot; 
         FIG. 18  is a flowchart for explaining the procedure of in-field active maintenance of the apparatus for housing a plug-in unit of the present application in which the insertion detecting device is not provided in the slot; and 
         FIG. 19  is a flowchart for explaining the procedure of in-field active maintenance of the apparatus for housing a plug-in unit of the present application in which the insertion detecting device is provided in the slot. 
     
    
    
     DESCRIPTION OF EMBODIMENT 
     In the package-misinsertion prevention mechanisms of related art, when the combination of a slot and the plug-in unit that is correctly inserted into and removed from the slot is to be changed after manufacturing, a worker has to manually replace the structure with a structure that enables replacing. A structure that enables replacing is generally provided in the back of the slot. When a plurality of slots are present, a problem occurs in that the number of man-hours in the replacement operation is large and mistakes may occur during the replacement. When misinsertion occurs as a result of mistakes during replacement, a problem also occurs in that time has to be used for recovery. 
     From one perspective, by attaching a permanent magnet on the plug-in unit side such that the polarity is based on the type of plug-in unit and providing an electromagnet of which the polarity is able to be changed on the plug-in unit side, the present application aims to provide an apparatus for housing a plug-in unit, which is capable of supporting changes in the type of plug-in unit. In addition, from another perspective, the present application aims to provide a plug-in unit that is able to be inserted into a slot of an apparatus for housing the plug-in unit, which includes, within the slot, an electromagnet of which the polarity is able to be changed. 
     Hereinafter, embodiments of the present disclosure will be described in detail based on specific examples, with reference to the accompanying drawings. In the examples described hereafter, a disk array is described as an apparatus for housing a plug-in unit (hereinafter also referred to as a housing apparatus) and a hard disk device is described as a plug-in unit. However, the present application is not limited thereto. In other words, as examples of the plug-in unit, a magnetic tape device, an optical disc device, a semiconductor memory device (SSD), and the like may be given, in addition to the hard disk device. The plug-in unit does not have to be a unit having the same physical shape as long as the plug-in unit is able to be inserted into a slot without changing the slot for the plug-in unit in the housing apparatus. 
       FIG. 2A  illustrates a disk array  1  as an example of the apparatus for housing a plug-in unit of the present application and a hard disk device  2  as an example of a plug-in unit that is housed in the housing apparatus. The disk array  1  is provided with a plurality of slots  10  that are elongated in a vertical direction and are each capable of housing therein the hard disk device  2 . For example,  80  slots  10  are provided. The hard disk device  2  that is housed in the slot  10  is able to be inserted and removed, and is replaceable. The hard disk device  2  includes a casing  20 . A hard disk device main body  21  is provided in the center portion of the casing  20 . A circuit board  23  on which a connector  24  is mounted as illustrated in  FIG. 2B  is provided on the insertion-end side of the hard disk device  2  to be inserted into the slot  10 . The circuit board  23  including the connector  24  is covered by a cover  22  as illustrated in  FIG. 2A . The hard disk device  2  is easily inserted into the slot  10 . 
       FIG. 2C  illustrates a misinsertion prevention mechanism PM 1  on the hard disk device-side of the first example of the present application, the misinsertion prevention mechanism PM 1  being provided on the insertion-end side of the hard disk device  2  to be inserted into the slot  10  illustrated in  FIG. 2B . Ribs  28  are provided on both sides of the circuit board  23  in the casing  20  on the insertion-end side of the hard disk device  2  to be inserted into the slot  10 . Two arms  26  are provided as movable pieces in the misinsertion prevention mechanism PM 1  on the hard disk device-side of the first example, each arm  26  being capable of swinging around a rotation shaft  25  that is provided in a projecting manner in the casing  20 . A permanent magnet  27  is embedded near the tip of each arm  26 . The rotation shaft  25  of the arm  26  on the upper side is provided in a projecting manner in the casing  20 , on the outer side of the rib  28 . The rotation shaft  25  of the arm  26  on the lower side is provided in a projecting manner in the casing  20 , in a section between the rib  28  and the circuit board  23 . A reason for this is to allow each arm  26  to be supported by a stopper  29  provided in the rib  28  by dropping naturally, because the hard disk device  2  is inserted into the disk array  1  in a vertically standing state as illustrated in  FIG. 2A . 
     The two arms  26  that are each supported by the stopper  29  provided in the rib  28  are parallel with the length direction of the hard disk device  2 . In this state, the polarity of the permanent magnet  27  embedded near the tip of the arm  26  is in a direction perpendicular to the length direction of the hard disk device  2 . Either of the N pole and the S pole of the permanent magnet  27  may face the outer side of the hard disk device  2 . 
     In the misinsertion prevention mechanism PM 1  on the hard disk device-side of the first example, the rotation shaft  25  of the arm  26  on the upper side is provided in a projecting manner in the casing  20 , on the outer side of the rib  28 . The rotation shaft  25  of the arm  26  on the lower side is provided in a projecting manner in the casing  20 , in a section between the rib  28  and the circuit board  23 . On the other hand, a misinsertion prevention mechanism PM 1 A of a first variation example of the first example illustrated in  FIG. 3A  is possible in which the rotation shafts  25  of the arms  26  on the upper and lower sides are both provided in a projecting manner in the casing  20 , on the outer sides of both ribs  28 . An misinsertion prevention mechanism PM 1 B of a second variation example of the first example illustrated in  FIG. 3A  is also possible in which the rotation shafts  25  of the arms  26  on the upper and lower sides are both provided in a projecting manner in the casing  20 , in the sections between the ribs  28  and the circuit board  23 . 
     However, in the misinsertion prevention mechanism PM 1 A in the first variation example of the first example, to enable the arm  26  on the lower side to come into contact with the stopper  29  provided in the rib  28 , a spring (not illustrated) for rotating the arm  26  upwards has to be used between the rotation shaft  25  and the arm  26 . In addition, in the misinsertion prevention mechanism PM 1 B in the second variation example of the first example, to enable the arm  26  on the upper side to come into contact with the stopper  29  provided in the rib  28 , a spring (not illustrated) for rotating the arm  26  upwards has to be used between the rotation shaft  25  and the arm  26 . Furthermore, when the hard disk device  2  is inserted horizontally in a disk array that is provided with laterally elongated slots, a spring may be provided between all rotation shafts  25  and arms  26  to enable each arm  26  to come into contact with the stopper  29  provided in the rib  28 . 
       FIGS. 4A and 4B  illustrate a first example of an internal structure of the slot  10  in the disk array  1  illustrated in  FIG. 2A . The internal structure corresponds with the hard disk device  2  including the misinsertion prevention mechanism PM 1  of the first example illustrated in  FIGS. 2B and 2C . The internal structure of the slot  10  illustrated in  FIGS. 4A and 4B  also correspond with the first variation example PM  1 A and the second variation example PM  1 B of the misinsertion prevention mechanism of the first example illustrated in  FIGS. 3A and 3B . 
     As illustrated in  FIGS. 4A and 4B , the slot  10  is formed so as to be surrounded by a single partition wall  11  and three side walls  11 A,  11 B, and  11 C. The portion of the slot  10  in which the side walls  11 A,  11 B, and  11 C are not present serves as an entry portion for the hard disk device  2 . In addition, the portion opposing the partition wall  11  is partitioned by the partition wall  11  of the adjacent slot  10 . The side wall  11 A is provided in the innermost portion of the slot  10 , and is provided with a cut-out section  11 K in the center portion. A back plate  13  is provided on the outer side of the side wall  11 A. A connector  14  that is mounted on the back plate  13  projects into the slot  10  from the cut-out section  11 K. Pins  12  that project into the slot  10  are provided on the side wall  11 A on both sides of the cut-out section  11 K. 
     In addition, the side wall  11 B serves as the bottom wall of the slot  10 , and the side wall  11 C serves as the ceiling wall of the slot  10 . Electromagnets  15  are provided so as to oppose each other on the inner sides of the side walls  11 B and  11 C, near the tips of the pins  12  provided in a projecting manner on the side wall  11 A. The direction of the magnetic fields generated by the electromagnets  15  is a direction perpendicular to the length direction of the slot  10 . The electromagnet  15  may or may not include an iron core. The structure of electromagnets is well known. Therefore, an illustration of the structure of the electromagnet  15  is omitted. In addition, a structure is also possible in which the electromagnets  15  are disposed on the outer sides of the side walls  11 B and  11 C and the iron cores (yokes) of the electromagnets  15  are project towards the inner sides of the side walls  11 B and  11 C. 
     Each electromagnet  15  is connected to a control unit (described hereafter) that is provided within the disk array  1  illustrated in  FIG. 2A . The electromagnet  15  receives a current from the control unit, thereby generating a magnetic field that is suitable for a valid hard disk device  2  that is inserted to the slot  10 . Therefore, in the control unit, control values prescribing the direction and the amount of the current to be provided to the electromagnet  15  are set in advance. The control values that are set in the control unit may be inputted by a host computer that is connected to the disk array  1 , an external input device, or the like. Therefore, when the type of hard disk device  2  to be inserted into a certain slot  10  is changed, the hard disk device  2  after the change in type is able to be validated by the control values prescribing the direction and the amount of the current to be provided to each electromagnet  15  in the slot  10  being changed. 
     In addition, in the misinsertion prevention mechanism PM 1  of the first example, the electromagnetic attractive force or the electromagnetic repulsive force between the permanent magnets  27  and the electromagnets  25 , and the length and weight of the arms  26  are set to enable the driving force for rotating the arms  26  to be obtained. Furthermore, in the misinsertion prevention mechanisms PM  1 A and PM  1 B of the first and second variation examples of the first example and when the hard disk device  2  is to be horizontally inserted, a spring constant that enables the arm  26  to be rotated by the electromagnet  15  is set for the spring provided between the rotation shaft  25  and the arm  26 . 
       FIG. 5A  illustrates a state immediately before the hard disk device  2  including the misinsertion prevention mechanism PM 1  of the first example is inserted into the slot  10  illustrated in  FIG. 4B . When the hard disk device  2  is inserted into the slot  10  from the state illustrated in  FIG. 5A , the state becomes that illustrated in  FIG. 5B . In the state illustrated in  FIG. 5B , the end portion of the hard disk device  2  is advancing forward but has not yet reached the electromagnets  15 . 
     In the state illustrated in  FIG. 5B , a situation may be considered in which the hard disk device  2  that has been inserted into the slot  10  is incorrect and unsuitable for the slot  10 . Energization of the electromagnets  15  is started before the hard disk device  2  is inserted or while the hard disk device  2  is advancing forward. At this time, even when the magnetic fields of the electromagnets  15  are applied to the permanent magnets  27  of the misinsertion prevention mechanism PM 1  provided in the end portion of the hard disk device  2 , the polarity of the permanent magnet  27  and the polarity of the magnetic field of the electromagnet  15  are not compatible. Consequently, the force for rotating the arms  26  is not able to be obtained. Therefore, the arms  26  of the misinsertion prevention mechanism PM 1  do not swing and remain in the initial state. In this state, when the hard disk device  2  is further advanced into the slot  10 , the tips of the two arms  26  come into contact with the tips of the two pins  12 . At the position at which the tips of the two arms  26  come into contact with the tips of the two pins  12 , the connector  24  provided in the hard disk device  2  is not coupled with the connector  14  provided in the back plate  14 . In this way, an incorrectly inserted hard disk device  2  is not connected to the back plate  13 . 
     Next, in the state illustrated in  FIG. 5B , a situation may be considered in which the hard disk device  2  that is inserted into the slot  10  is correct and suitable for the slot  10 . Energization of the electromagnets  15  is started before the hard disk device  2  is inserted or while the hard disk device  2  is advancing forward. In this instance, as illustrated in  FIG. 6A , the permanent magnets  27  of the misinsertion prevention mechanism PM 1  provided at the end portion of the hard disk device  2  are affected by the magnetic fields of the electromagnets  15 . Therefore, each arm  26  rotates and moves to a retreated position. A reason for this is that, in the correct hard disk device  2 , because the polarity of the permanent magnet  27  and the polarity of the electromagnetic field of the electromagnet  15  are compatible, the force that enables the arm  26  in the misinsertion prevention mechanism PM 1  to be rotated by the electromagnet  15  is able to be obtained. 
     The directions of the magnetic fields generated by the electromagnets  15  when the hard disk device  2  that is inserted into the slot  10  is correct and suitable for the slot  10  will be described with reference to  FIG. 7A . The electromagnet  15  on the upper side generates a magnetic field that attracts the permanent magnet  27  within the arm  26  on the upper side. Conversely, the electromagnet  15  on the lower side generates a magnetic field that repels the permanent magnet  27  within the arm  26  on the lower side. As a result, the arm  26  on the upper side is attracted to the electromagnet  15  on the upper side, and the arm  26  on the lower side moves in a direction away from the electromagnet  15  on the lower side. 
     When the hard disk device  2  is further inserted into the slot  10  from the state illustrated on the right side in  FIG. 7A , the state illustrated in  FIG. 6A  is reached, followed by the state illustrated in  FIG. 6B . The connector  24  attached to the circuit board  23  is coupled with the connector  14  attached to the back plate  13 . In other words, the arms  26  on the upper and lower sides provided in the hard disk device  2  rotate and retreat from the pins  12  by the magnetic fields of the electromagnets  15 . Therefore, the pins  12  enter the spaces from which the arms  26  have moved, and the hard disk device  2  may be moved to the innermost portion of the slot  10 . 
     The control values for the electromagnets  15  provided within the slot  10  are set such that, for the hard disk device  2  including the misinsertion prevention mechanism PM 1 , the electromagnet  15  on the upper side generates a magnetic field that attracts the permanent magnet  27 , and the electromagnet  15  on the lower side generates a magnetic field that repels the permanent magnet  27 . On the other hand, when the hard disk device  2  includes the misinsertion prevention mechanism PM  1 A of the first variation example of the first example, as illustrated in  FIG. 7B , the control values are set such that both electromagnets  15  on the upper and lower sides generate magnetic fields that attract the permanent magnets  27 . In addition, when the hard disk device  2  includes the misinsertion prevention mechanism PM 1 B of the second variation example of the first example, the control values are set such that both electromagnets  15  on the upper and lower sides generate magnetic fields that repel the permanent magnets  27  (not illustrated). 
     As described above, in the disk array  1  in which the hard disk device  2  including the misinsertion prevention mechanism PM 1  of the first example is housed in the slot  10 , the polarity of the magnetic field generated by the electromagnet  15  is determined in advance based on the polarity of the permanent magnet  27  embedded in the arm  26 . Therefore, when a hard disk device  2  is inserted into the slot  10  of which the polarity of the permanent magnet  27  is not compatible with the polarity of the electromagnet  15 , the arm  26  does not move and the hard disk device  2  is not able to be inserted into the slot  10 . On the other hand, even when the type of hard disk device  2  to be inserted into the slot  10  of the disk array  1  is changed, the polarities of the electromagnets  15  within the slot  10  are able to be changed by the control values set in the control units being changed, thereby enabling changes in type to be supported. 
       FIG. 8A  illustrates a configuration of a misinsertion prevention mechanism PM 2  of a second example of the present application provided in the hard disk device  2  and a configuration of a corresponding slot  10  in the disk array  1 . The misinsertion prevention mechanism PM 2  is provided on the lower side and the upper side of the insertion end of the hard disk device  2 , or on either the lower side or the upper side. The movable piece in the misinsertion prevention mechanism PM 2  is a slide rod  31  that emerges from a rod case  30 . A permanent magnet  32  is embedded in the slide rod  31 . When the misinsertion prevention mechanism PM 2  is provided on the lower side of the insertion end of the hard disk device  2 , the slide rod  31  projects from the rod case  30  by dropping naturally. 
     On the other hand, when the misinsertion prevention mechanism PM 2  is provided on the upper side of the insertion end of the hard disk device  2 , the slide rod  31  ordinarily sinks into the rod case  30  by dropping naturally. Therefore, when the misinsertion prevention mechanism PM 2  is provide on the upper side of the insertion end of the hard disk device  2 , a spring is inserted within the rod case  30  to enable the tip of the slide rod  31  to project from the rod case  30 .  FIG. 8A  illustrates only the misinsertion prevention mechanism PM 2  on the lower side of the insertion end of the hard disk device  2 . An illustration of the misinsertion prevention mechanism PM 2  provided on the upper side of the insertion end of the hard disk device  2  is omitted. 
     Within the slot  10  corresponding with the hard disk device  2  including the misinsertion prevention mechanism PM 2  of the second example, the pin  12  is provided on the side wall  11 A in the innermost portion so as to come into contact with the side surface of the slide rod  31  that is projecting from the rod case  30 . The second example is similar to the above-described first example in that a cut-out section is provided in the center portion of the side wall  11 A and a connector that is mounted on the back plate  13  provided on the outer side of the side wall  11 A projects into the slot  10  from the cut-out section. However, illustration of the cut-out section and the connector are omitted in  FIG. 8A . 
     In addition, the electromagnet  15  is provided on the inner side of the side wall  11 B that serves as the bottom wall of the slot  10 , near the tip of the pin  12  provided in a projecting manner on the side wall  11 A. The direction of the magnetic field generated by the electromagnet  15  is a direction perpendicular to the length direction of the slot  10 . The electromagnet  15  may or may not include an iron core. The end portion of the electromagnet  15  is in a position that is not in contact with the side surface of the slide rod  31  that projects from the rod case  30 . In the state illustrated in  FIG. 8A , the end portion of the hard disk device  2  has not yet reached the electromagnet  15 . 
     Here, in the state illustrated in  FIG. 8A , a situation may be considered in which the hard disk drive  2  that is inserted into the slot  10  is incorrect and not suitable for the slot  10 . Energization of the electromagnet  15  is started before the hard disk device  2  is inserted or while the hard disk device  2  is advancing forward. At this time, even when the magnetic field of the electromagnet  15  is applied to the permanent magnet  32  of the misinsertion prevention mechanism PM 2  provided in the end portion of the hard disk device  2 , the polarity of the permanent magnet  32  and the polarity of the magnetic field of the electromagnet  15  are not compatible. Consequently, the force for moving the slide rod  31  into the rod case  30  is not able to be obtained. Therefore, the slide rod  31  in the misinsertion prevention mechanism PM 2  remains in the initial state in which the slide rod  31  is projecting from the rod case  30 . In this state, when the hard disk device  2  is further advanced into the slot  10 , as illustrated in  FIG. 8B , the side surface of the slide rod  31  comes into contact with the tip of the pin  12 . At the position at which the side surface of the slide rod  31  comes into contact with the tip of the pin  12 , the connector provided in the hard disk device  2  is not coupled with the connector provided in the back plate  14 . In this way, an incorrectly inserted hard disk device  2  is not connected to the back plate  13 . 
     Next, in the state illustrated in  FIG. 8A , a situation may be considered in which the hard disk device  2  that is inserted into the slot  10  is correct and suitable for the slot  10 . Energization of the electromagnet  15  is started before the hard disk device  2  is inserted or while the hard disk device  2  is advancing forward. In this instance, as illustrated in  FIG. 8C , the permanent magnet  32  of the misinsertion prevention mechanism PM 2  provided at the end portion of the hard disk device  2  is affected by the magnetic field of the electromagnet  15 . The direction of the magnetic field generated by the electromagnet  15  at this time is a direction enabling the slide rod  31  to sink into the rod case  30 , or in other words, a direction that repels the permanent magnet  32 . As a result, the slide rod  31  sinks into the rod case  30  by the magnetic field of the electromagnet  15 . 
     When the hard disk device  2  is further inserted into the slot  10  from the state illustrated in  FIG. 8C , because the pin  12  does not come into contact with the slide rod  31  that has sunk into the rod case  30 , the hard disk device  2  may be moved to the innermost portion of the slot  10 . The operation is also similar for the misinsertion prevention mechanism PM 2  that is provided on the upper side of the insertion end of the hard disk device  2 . 
     In the disk array  1  in which the hard disk device  2  including the misinsertion prevention mechanism PM 2  is housed in the slot  10  as described above, the polarity of the magnetic field generated by the electromagnet  15  is determined in advance by a control value, based on the polarity of the permanent magnet  32  embedded in the slide rod  31 . Therefore, when a hard disk device  2  is inserted into the slot  10  of which the polarity of the permanent magnet  32  is not compatible with the polarity of the electromagnet  15 , the slide rod  31  does not sink into the rod case  30 , and the hard disk device  2  is not able to be inserted into the slot  10 . On the other hand, even when the type of hard disk device  2  changes and the polarity of the permanent magnet  32  changes, the polarity of the electromagnet  15  within the slot  10  is able to be changed by the control value set in the control unit being changed. Therefore, changes in type may be supported. 
       FIG. 9A  illustrates a configuration of a misinsertion prevention mechanism PM 3  of a third example of the present application provided in the hard disk device  2  and a configuration of a corresponding slot  10  in the disk array  1 . The misinsertion prevention mechanism PM 3  is provided on the lower side and the upper side of the insertion end of the hard disk device  2 , or on either the lower side or the upper side. The movable piece in the misinsertion prevention mechanism PM 3  is a cam  42  that swings in relation to a cam shaft  41  that is provided in a projecting manner on a cam base  40 . A slit  43  is provided in a length-axis direction of the cam  42 . A permanent magnet  47  is embedded within the cam  42  below the slit  43 . 
     In addition, a first post  45  and a second post  46  are provided in the swinging range of the cam  42  to restrict the swinging range of the cam  42 . When the misinsertion prevention mechanism PM 3  is provided on the lower side of the insertion end of the hard disk device  2 , the cam  42  drops naturally and is held by the first post  45 . In a state in which the cam  42  is being held by the first post  45 , the slit  43  that is provided in the cam  42  is parallel with the length direction of the slot  10 . In a state in which the cam  42  is in contact with the second post  46 , the slit  43  has a predetermined angle in relation to the length direction of the slot  10 . 
     On the other hand, when the misinsertion prevention mechanism PM 3  is provided on the upper side of the insertion end of the hard disk device  2 , a misinsertion prevention mechanism PM 3  similar to the misinsertion prevention mechanism PM 3  illustrated in  FIG. 9A  may be provided on the upper side of the insertion end of the hard disk device  2 .  FIG. 9A  illustrates only the misinsertion prevention mechanism PM 3  on the lower side of the insertion end of the hard disk device  2 . An illustration of the misinsertion prevention mechanism PM 3  provided on the upper side of the insertion end of the hard disk device  2  is omitted. 
     Within the slot  10  corresponding with the hard disk device  2  including the misinsertion prevention mechanism PM 3  of the third example, the pin  12  is provided on the side wall  11 A in the innermost portion so as to enter the slit  43  that is provided in the cam  42 . The third example is similar to the above-described first example in that a cut-out section is provided in the center portion of the side wall  11 A and a connector that is mounted on the back plate  13  provided on the outer side of the side wall  11 A projects into the slot  10  from the cut-out section. However, illustration of the cut-out section and the connector are omitted in  FIG. 9A . 
     In addition, the electromagnet  15  is provided on the inner side of the side wall  11 B that serves as the bottom wall of the slot  10 , near the tip of the pin  12  provided in a projecting manner on the side wall  11 A. The direction of the magnetic field generated by the electromagnet  15  is a direction perpendicular to the length direction of the slot  10 . The electromagnet  15  may or may not include an iron core. The end portion of the electromagnet  15  is in a position that is not in contact with the first post  45  that is provided in a projecting manner on the cam base  40 . In the state illustrated in  FIG. 9A , the end portion of the hard disk device  2  has not yet reached the electromagnet  15 . 
     Here, in the state illustrated in  FIG. 9A , a situation may be considered in which the hard disk device  2  that is inserted into the slot  10  is correct and suitable for the slot  10 . Energization of the electromagnet  15  is started before the hard disk device  2  is inserted or while the hard disk device  2  is advancing forward. At this time, as illustrated in  FIG. 9B , even when the magnetic field of the electromagnet  15  is applied to the permanent magnet  47  of the misinsertion prevention mechanism PM 3 , the polarity of the permanent magnet  47  and the polarity of the magnetic field of the electromagnet  15  are not compatible. Consequently, the force for rotating the cam  42  is not able to be obtained. Therefore, the cam  42  in the misinsertion prevention mechanism PM 3  does not swing and remains in the initial state in which the cam  42  is being held by the first post  45 . 
     Therefore, when the hard disk device  2  is further inserted into the slot  10  from the state illustrated in  FIG. 9A , the pin  12  that is provided in a projecting manner on the side wall  11 A enters the slit  43  that is provided in the cam  42 . As a result, the hard disk device  2  is may be moved to the innermost portion of the slot  10 . The operation is also similar for the misinsertion prevention mechanism PM 3  that is provided on the upper side of the insertion end of the hard disk device  2 . 
     Next, in the state illustrated in  FIG. 9A , a situation may be considered in which the hard disk drive  2  that is inserted into the slot  10  is incorrect and not suitable for the slot  10 . Energization of the electromagnet  15  is started before the hard disk device  2  is inserted or while the hard disk device  2  is advancing forward. At this time, when the magnetic field of the electromagnet  15  is applied to the permanent magnet  47  of the misinsertion prevention mechanism PM 3 , because the polarity of the permanent magnet  47  and the polarity of the magnetic field of the electromagnet  15  are compatible, the force for rotating the cam  42  is able to be obtained. In other words, the electromagnet  15  generates a magnetic field having a polarity that is opposite of the polarity of the permanent magnet  47 . As a result, the cam  42  rotates upwards around the cam shaft  41  until the outer peripheral portion of the cam  42  comes into contact with the second post  46 . When the hard disk device  2  is further inserted into the slot  10  in a state in which the outer peripheral portion of the cam  42  is in contact with the second post  46 , as illustrated in  FIG. 9C , the outer peripheral portion of the cam  42  comes into contact with the tip of the pin  12 . The hard disk device  2  is not able to be inserted any further. At the position at which the outer peripheral portion of the cam  42  is in contact with the tip of the pin  12 , the connector provided in the hard disk device  2  is not coupled with the connector provided in the back plate  14 . In this way, an incorrectly inserted hard disk device  2  is not connected to the back plate  13 . 
     In the disk array  1  in which the hard disk device  2  including the misinsertion prevention mechanism PM 3  is housed in the slot  10  as described above, the polarity of the magnetic field generated by the electromagnet  15  is determined in advance based on the polarity of the permanent magnet  47  in the cam  42 , by a control value being set in the control unit. Therefore, when a hard disk device  2  is inserted into the slot  10  of which the polarity of the permanent magnet  47  is not compatible with the polarity of the electromagnet  15 , the cam  42  rotates around the cam shaft  41 , and the hard disk device  2  is not able to be inserted into the slot  10 . On the other hand, even when the type of hard disk device  2  changes, the polarity of the electromagnet  15  within the slot  10  is able to be changed by the control value set in the control unit being changed. Therefore, changes in type are able to be supported. 
       FIG. 10A  illustrates an outer appearance of the replaceable unit  5  such as a hard disk device that is housed in a disk array. In the above-described examples of the hard disk device  2 , a total of two movable pieces  51  and  52  that each include a permanent magnet are mounted, one on the lower end side and one on the upper end side of the replaceable unit  5 , in a position in the replaceable unit  5  indicated by a virtual line XB, as illustrated in  FIG. 10B . In this instance, there are four combinations of the upper electromagnet, the lower electromagnet, the upper permanent magnet, and the lower permanent magnet, as illustrated in  FIG. 10E . Four replaceable unit  5  patterns are able to be created. 
     On the other hand, a total of three movable pieces  51  to  53  that each include a permanent magnet may be mounted, one movable piece  51  on the lower end side and two movable pieces  52  and  53  on the upper end side of the replaceable unit  5 , in a position in the replaceable unit  5  indicated by a virtual line XC, as illustrated in  FIG. 10C . In this instance, there are eight combinations of the upper electromagnets, the lower electromagnet, the upper permanent magnets, and the lower permanent magnet, as illustrated in  FIG. 10F . Eight replaceable unit  5  patterns are able to be created. 
     Furthermore, a total of four movable pieces  51  to  54  that each include a permanent magnet may be mounted, two movable pieces  51  and  52  on the lower end side and two movable pieces  53  and  54  on the upper end side of the replaceable unit  5 , in a position in the replaceable unit  5  indicated by a virtual line XD, as illustrated in  FIG. 10D . In this instance, there are  16  combinations of the upper electromagnets, the lower electromagnets, the upper permanent magnets, and the lower permanent magnets, as illustrated in  FIG. 10F . Sixteen replaceable unit  5  patterns are able to be created. When the number of movable pieces is increased, the types of replaceable unit  5  may be further increased. The polarities generated by the electromagnets disposed in positions corresponding with the movable pieces  51  to  54  that each include a permanent magnet may be set by control values being set in the control units. 
       FIG. 11A  illustrates a configuration of a misinsertion prevention mechanism PM 4  of a fourth example of the present application provided in the replaceable unit  5  and an internal configuration of a corresponding slot  10 . The misinsertion prevention mechanism PM 4  is configured by three misinsertion prevention mechanisms PM 2  of the second example being aligned in series at differing heights in the length direction of the slot  10 . Therefore, the misinsertion prevention mechanism PM 4  includes three rod cases  30 A,  30 B, and  30 C, and slide rods  31 A,  31 B, and  31 C that are housed within the rod cases  30 A,  30 B, and  30 C. In the fourth example, the misinsertion prevention mechanism PM 4  is only provided on the lower end side of the replaceable unit  5 . However, the misinsertion prevention mechanism PM 4  may also be provided on the upper end side. 
     Meanwhile, the pin  12  that is provided in a projecting manner on the side wall  11 A, three electromagnets  15 A,  15 B, and  15 C, and an attachment base  16  for the two electromagnets  15 A and  15 B are included within the slot  10 . The electromagnet  15 C is provided on the side wall  11 B. The attachment base  16  is provided with a stopper  16 C that comes into contact with the side surface of the slide rod  31 C that projects from the rod case  30 C and a stopper  16 B that comes into contact with the side surface of the slide rod  31 B that projects from the rod case  30 B. In addition, the pin  12  is provided in a position that enables the pin  12  to come into contact with the side surface of the slide rod  31 A that projects from the rod case  30 A. The electromagnet  15 B is positioned between the stopper  16 B and the stopper  16 C, and the electromagnet  15 A is positioned between the stopper  16 B and the pin  12 . 
     Energization of the electromagnets  15 A,  15 B, and  15 C is started before the replaceable unit  5  is inserted or while the replaceable unit  5  is advancing forward. First, when the polarities of all permanent magnets are not compatible with the polarities of the magnetic fields of the electromagnets  15 A,  15 B, and  15 C, even when the magnetic fields of the electromagnets  15 A,  15 B, and  15 C are applied, the force is not able to be obtained to move any of the slide rods  31 A,  31 B, and  31 C into the rod cases  30 A,  30 B, and  30 C. At this time, when the replaceable unit  5  is inserted into the slot  10 , as illustrated in  FIG. 11B , the pin  12  comes into contact with the side surface of the slide rod  31 A that projects from the rod case  30 A. In addition, the stopper  16 B comes into contact with the side surface of the slide rod  31 B that projects from the rod case  30 B, and the stopper  16 C comes into contact with the side surface of the slide rod  31 C that projects from the rod case  30 C. 
     Next, when the polarities of all permanent magnets are not compatible with the polarity of the magnetic field of any one of the electromagnets  15 A,  15 B, or  15 C, even when the magnetic fields of the electromagnets  15 A,  15 B, and  15 C are applied, the force is not able to be obtained for moving one of the slide rods  31 A,  31 B, and  31 C into the respective rod cases  30 A,  30 B, and  30 C. Therefore, the slide rod  31 A,  31 B, or  31 C of which the polarity of the permanent magnet is not compatible with the polarity of the magnetic field of the electromagnet  15 A,  15 B, or  15 C does not move into the rod case  30 A,  30 B, or  30 C, and comes into contact with a stopper  16 B or  16 C, or the pin  12 . Therefore, the replaceable unit  5  is not able to be fully inserted.  FIG. 12A  illustrates when only the polarity of the permanent magnet in the slide rod  31 B is not compatible with the polarity of the magnetic field of the electromagnet  15 B. At this time, only the slide rod  31 B that opposes the electromagnet  15 B does not move into the rod case  30 B and comes into contact with the stopper  16 B. Therefore, the replaceable unit  5  is not able to be fully inserted. 
     Finally, when the polarities of all permanent magnets are compatible with the polarities of the magnetic fields of the electromagnets  15 A,  15 B, and  15 C, when the magnetic fields of the electromagnets  15 A,  15 B, and  15 C are applied, as illustrated in  FIG. 12B , all slide rods  31 A,  31 B, and  31 C sink into the rod cases  30 A,  30 B, and  30 C. When the replaceable unit  5  is inserted into the slot  10  in this state, the replaceable unit  5  is able to move to the innermost portion of the slot  10 . 
       FIG. 13A  illustrates an example in which a position sensor  6  that is an insertion detecting device for the hard disk device  2  is provided in the slot  10  illustrated in  FIG. 4B  of the present application. The position sensor  6  is provided in substantially the center portion of the slot  10 . As illustrated in  FIG. 13B , the position sensor  6  includes a sensor arm  62  on a rotation shaft  61  that is provided on a sensor base  60 , a contact  63  being provided in the end portion of the sensor arm  62 , and a first electrode  64  and a second electrode  65  that are provided on the sensor base  60 . When the hard disk device  2  is inserted into the slot  10 , the sensor arm  62  is pressed and rotates around the rotation shaft  61 . The contact  63  provides conduction between the first electrode  64  and the second electrode  65 . As a result, electric current flows between the first electrode  64  and the second electrode  65 , and an electromagnet connected to the first electrode  64  and the second electrode  65  is turned on. 
       FIG. 14  is a block circuit diagram of an overall configuration of the apparatus for housing a plug-in unit (disk array  1 ) of the present application in which the position sensor  6  serving as an insertion detecting device is not provided in the slot  10 . A plurality of connectors  24  are mounted on the back plate  13  that is provided within the disk array  1 . A large portion of the connector  24  projects into the slot  10  of the disk array  1 . In addition, a control unit  3  is connected to the connector  24  via a connector  34 . A power supply unit  4  is also connected to the connector  24  via a connector  44 . An input device  8  is connected to the control unit  3 . The power supply unit  4  is connected to an alternating current (AC) power source or a direct current (DC) power source via an input terminal  9 . The electromagnet  15  provided within the slot  10  is connected to the control unit  3  via the back plate  13 . Energization taking into consideration the polarity of the magnetic field generated by the electromagnet  15  is performed based on a control value set in the control unit  3 . 
       FIG. 15  is a block circuit diagram of an overall configuration of the apparatus for housing a plug-in unit (disk array  1 ) of the present application in which the position sensor  6  serving as an insertion detecting device is provided in the slot  10 . Excluding the position sensor  6 , the structure of the disk array  1  provided with the position sensor  6  is similar to that of the disk array  1  described with reference to  FIG. 14 . Therefore, the same constituent components are given the same reference symbols, and descriptions thereof are omitted. 
       FIG. 16  is a flowchart for explaining the procedure in a factory initial setting process that is performed at a factory of the disk array of the present application in which a position sensor is not provided in a slot, such as that illustrated in  FIG. 14 . In an initial state indicated at step  161 , the power is off and no units are mounted. At subsequent step  162 , device power is turned on. At step  163 , the default setting of the unit mounting position is set or individual settings are written in a memory area of the control unit. In this setting procedure, for example, a control table or the like is written in the memory area of the control unit, the control table in which the control values for controlling the direction of the current and the amount of current to be sent to the electromagnet are written in correspondence with the types of hard disk devices to be inserted into the respective slots. At subsequent step  164 , all electromagnets in the disk array are turned on. At step  165 , the hard disk devices that are plug-in units are mounted in all of the slots. At this time, when a hard disk device that is mounted in a slot is unsuitable, the polarity of the permanent magnet within the hard disk device and the polarity of the electromagnet within the slot that is based on the above-described control value are not compatible. Therefore, the unsuitable hard disk device is not able to be inserted into the slot. 
     At step  166 , when a normal operation signal (such as a READY signal) of the mounted plug-in unit (hard disk device) is received, energization of the electromagnet in the slot is turned off. At step  167 , whether or not the desired number of plug-in units (hard disk devices) has been mounted is determined. When the desired number of plug-in units (hard disk devices) has not been mounted, the procedure at step  165  to step S 167  is repeated. Then, when determined at step  167  that the desired number of plug-in units (hard disk devices) has been mounted, the flow proceeds to step  168  and the device power is turned off. At step  169 , the disk array is shipped. 
       FIG. 17  is a flowchart for explaining the procedure in the factory initial setting process that is performed at a factory of the disk array of the present application in which a position sensor is provided in a slot, such as that illustrated in  FIG. 15 . The steps that are the same as those in  FIG. 16  are described using the same step numbers. In the initial state indicated at step  161 , the power is off and no units are mounted. At subsequent step  162 , device power is turned on. At step  163 , the default setting of the unit mounting position is set or individual settings are written in a memory area of the control unit. In this setting procedure, for example, a control table or the like is written in the memory area of the control unit, the control table in which the control values for controlling the direction of the current and the amount of current to be sent to the electromagnet are written in correspondence with the types of hard disk devices to be inserted into the respective slots. At subsequent step  165 , the hard disk devices that are plug-in units are mounted in all of the slots. 
     As indicated at step  171 , the position sensor is turned on when the hard disk device is mounted. Therefore, the electromagnet in the slot in which the hard disk device is mounted is turned on. At this time, when the hard disk device that is being mounted in the slot is unsuitable, the polarity of the permanent magnet within the hard disk device and the polarity of the electromagnet within the slot that is based on the above-described control value are not compatible. Therefore, the unsuitable hard disk device is not able to be inserted into the slot. 
     At step  166 , when a normal operation signal (such as a READY signal) of the mounted plug-in unit (hard disk device) is received, energization of the electromagnet in the slot is turned off. At step  167 , whether or not the desired number of plug-in units (hard disk devices) has been mounted is determined. When the desired number of plug-in units (hard disk devices) has not been mounted, the procedure at steps  165 ,  171 ,  166 , and  167  is repeated. Then, when determined at step  167  that the desired number of plug-in units (hard disk devices) has been mounted, the flow proceeds to step  168  and the device power is turned off. At step  169 , the disk array is shipped. Whereas all of the electromagnets are turned on at step  164  in the control procedure illustrated in  FIG. 16 , the control procedure illustrated in  FIG. 17  differ in that the electromagnets are turned on one by one by the position sensors at step  171 . 
       FIG. 18  is a flowchart for explaining the procedure of in-field active maintenance of the disk array of the present application when the position sensor is not provided in the slot. Active maintenance refers to an operation for replacing a hard disk device for maintenance purposes, while the disk array remains in operation. At step  181 , whether or not the device power has to be turned off is determined. When the device power has to be turned off, active maintenance is not performed. Therefore, the flow proceeds to step  182  and the routine is completed without active maintenance being performed. 
     When the device power does not have to be turned off, the flow proceeds to step  183 , and whether or not hot swapping of a hard disk device that is a plug-in unit is desired is determined. When hot swapping of a hard disk device is not desired, the routine is completed without active maintenance being performed. On the other hand, when determined at step  183  that hot swapping is desired, the flow proceeds to step  184 , and a slot in which the hot swapping is to be performed and the hard disk device to be newly inserted into the slot are designated through use of a graphical interface (GUI) displayed in a display device of an input device in the disk array. 
     The hot swapping includes active maintenance in which a running hard disk device is removed (made blank) and active maintenance in which a running hard disk device is replaced with a dummy. Therefore, at step  185 , whether a change is performed to make the hard disk device to be replaced blank, or a change is performed to replace the hard disk device with a dummy is determined. When the change to the hard disk device to be replaced is determined to be a change to blank or a change to a dummy, the flow proceeds to step  186  and the hard disk device to be replaced (old unit) is removed. At step  187 , a blank or a dummy unit is mounted and the routine is completed. The dummy unit is not connected to the disk array. Therefore, the dummy is able to be inserted into the slot regardless of the electromagnet being on or off and regardless of the polarity of the magnetic field when the electromagnet is on. 
     On the other hand, when determined at step  185  that the change is not a change to blank or to a dummy, the flow proceeds to step  188 . At step  188 , all of the electromagnets of in the disk array are turned on. At step  189 , the hard disk device to be replaced (old unit) is removed. At step  190 , a new hard disk device (new unit) is mounted. This procedure includes a procedure for removing a plurality of hard disk devices to be replaced (old units) and mounting a plurality of new hard disk devices (new units). 
     At this time, when a hard disk device to be mounted in a slot is unsuitable, the polarity of the permanent magnet within the hard disk device and the polarity of the electromagnet within the slot are not compatible. Therefore, the unsuitable hard disk device is not able to be inserted into the slot. At subsequent step  191 , when a normal operation signal (such as a READY signal) of the mounted hard disk device is received, energization of the electromagnet in the slot is turned off and active maintenance is completed. 
       FIG. 19  is a flowchart for explaining the procedure of in-field active maintenance of the disk array of the present application when the position sensor is provided in the slot. The difference between the active maintenance procedure illustrated in  FIG. 19  and the active maintenance procedure illustrated in  FIG. 18  is the procedure performed when the change is not a change to blank or a change to a dummy at step  185 . Therefore, the steps that are the same as those in  FIG. 18  are given the same step number, and the descriptions thereof are omitted. 
     At step  185 , when determined that the change is not a change to blank or a change to a dummy, the flow proceeds to step  192 . At step  192 , the hard disk device to be replaced (old unit) is removed. At step  193 , a new hard disk device (new unit) is mounted in the slot. This procedure includes a procedure for removing a plurality of hard disk devices to be replaced (old units) and mounting a plurality of new hard disk devices (new units). 
     When a new hard disk device (new unit) is inserted into a slot at step  193 , the position sensor is turned on when the hard disk device is mounted. Therefore, at step  194 , the electromagnet in the slot in which the hard disk device is mounted is turned on. At this time, when a hard disk device to be mounted in a slot is unsuitable, the polarity of the permanent magnet within the hard disk device and the polarity of the electromagnet within the slot are not compatible. Therefore, the unsuitable hard disk device is not able to be inserted into the slot. At subsequent step  195 , when a normal operation signal (such as a READY signal) of the mounted hard disk device is received, energization of the electromagnet in the slot is turned off and active maintenance is completed. Whereas all of the electromagnets are turned on at step  188  in the control procedure illustrated in  FIG. 18 , the control procedure illustrated in  FIG. 19  differ in that the electromagnets are turned on one by one by the position sensors at step  194 . 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiment of the present invention has been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.