In recent years there has been a demand for magnetic disk drives with an increased capacity and a reduced size.  Magnetic disk drives having a flat-external dimension of 1.0 inch are brought to the commercial stage in the level of current products. 
The conventional magnetic disk drives have an in-hub motor configuration or under-hub motor configuration.  The in-hub motor is configured such that a magnetic circuit including a rotor and a stator is disposed inside the center hub of a spindle on which disks are stacked, which is now in most widespread use.  The under-hub motor is configured such that a magnetic circuit including a rotor and a stator is disposed nearer a base than a hub portion on which disks are stacked. 
The configuration described in Japanese Patent Laid-open No.  6-68592 is an example of disk drive configurations using the in-hub motor. 
As shown in FIG.  2 of the above publication, this configuration employs a shaft-fixed type in-hub motor, in which a hub is disposed on a shaft secured to a base by way of bearings, magnets constituting a rotor are disposed inside the hub, and a stator is disposed on the base. 
It is Japanese Patent Laid-open No.  7-182771 that discloses a magnetic disk drive configuration using the under-hub motor. 
As shown in FIG.  2 of this publication, this configuration employs a shaft-fixed type under-hub motor, in which a hub is disposed on a fixed shaft secured to a base by way of bearings, magnets constituting a rotor are disposed below the hub, and a stator is disposed on the base. 
There has been an increase in demand for a low-profile magnetic disk drive in recent years.  Although the overall drive can be thinned in principle by making individual components thin, these components include ones impossible to make thin simply.  One of them is an enclosure. 
Since a shaft is a central axis around which a hub rotates irrespective of a shaft-rotary type or the shaft-fixed type, a portion of a base positioned near the shaft needs a certain thickness so as to withstand a large force exerted on the shaft.  In addition, the base needs mass that overcomes a force due to the rotation of the hub. 
Therefore, it is preferable for a characteristic of a recess provided on the base that the recess be provided at a position away from the shaft.  In addition, preferably as the recess is provided at a position nearer the shaft, the area of the recess should be made smaller. 
However, since the stator is formed so as to cover the shaft in the above prior art, if it is intended to make a low-profile device by lowering the position of the stator to the base side, it is inevitable to form the recess circularly or annularly in the base around the shaft.  That is, the prior art has not taken into consideration that the enclosure is made thinner with care taken to the durability thereof. 
In addition, the prior art simple describes that the stator is mounted on the base; no consideration is given to the arrangement among the stator and other members, more specifically, the relation between the stator and the enclosure facing the base; furthermore, a change in the strength of the enclosure itself is not taken into consideration. 
That is, an object of the present invention is to make a magnetic disk drive thinner with the durability of an enclosure improved. 
Besides the enclosure, components impossible to make smaller include a spindle motor and a battery. 
For a magnetic disk, it is necessary to retract a magnetic head from the surface of the magnetic disk, where the magnetic head is placed, before rotation of the spindle motor is stopped.  When this is carried out in a normal operation, an external power source is used for it; however, in the event of unplanned power interruption, the external power source cannot be used. 
In order to perform such operation, an internal power source connected to a motor (VCM) for shifting the magnetic head is needed, and in general a battery performs the function.  A large-scaled battery with a large capacity is inevitably selected because of a battery capacity necessary for the battery. 
If small batteries are dispersedly mounted on a board, this poses problems that performance is degraded due to wiring resistance, a mounting area is increased, and so forth.  Concrete measures to solve the problems are not taken into consideration. 
Accordingly, another object of this invention is to prevent a deterioration in the durability of an enclosure as well as to reduce a battery mounting area, and to downsize a magnetic disk drive as well as to thin the drive down.