1.  Field of the Invention
The present invention relates to a data storage system and a method of controlling the data storage system, and more particularly, to an optimal track zero determination method used in a disk drive, and a disk drive using the method. 
2.  Description of the Related Art
Related techniques are disclosed in U.S. Pat.  No 5,796,546 and Korean Patent Laid-open No.  1999-061745. 
In a seek control apparatus for a disk drive disclosed in U.S. Pat.  No.  5,796,546, defects on a disk is detected and extracted, and target track position information is changed by using extracted defect position, so that any defects cannot exist along a seek path.  In a logical track zero position determination method used in a disk drive disclosed in Korean Patent Laid-open No.1999-061745, a logical track zero position is determined by using a measured expansion coefficient of an O-ring and a bias force. 
In general, a hard disk drive, an example of a data storage apparatus, contributes to computer system operation by reading and/or writing data from and/or to a disk by using a magnetic head.  As the hard disk drive has a tendency to have a high capacity, a high density, and a compact size, a bit per inch (BPI) measurement in a rotational direction, and a track per inch (TPI) measurement in a radial direction of the hard disk drive become large.  As a result, there is a demand for a highly accurate mechanism in the hard disk drive. 
The hard disk drive comprises a head disk assembly (HDA) and a printed circuit board (PCB) assembly electrically controlling the HDA to read and/or write data from and/or to the disk.  The HDA comprises a disk on which data is recorded, a head to read and/or write the data from and/or to the disk, a spindle motor rotating the disk, an actuator arm moving the head, a voice coil motor (VCM), and an outer crash stop (OCS) limiting a displacement of the actuator arm.  Here, the OCS is an example of mechanical buffering unit preventing the head from moving to a position of the disk where servo information is not recorded. 
As the hard disk drive has a tendency to have a high recording density, there is a problem in that it takes a long time to record the servo information on the disk. 
To solve the problem, one solution that has been developed is an offline servo track write (Offline STW) method for recording the servo information on a plurality of disks at one time by using an external apparatus before the disk is assembled. 
But, in a case where the Offline STW method is used, a center of a disk is not aligned to the shaft of the spindle motor due to a mechanical error of the OCS, as well as a mechanical error occurring at the time when the disk is assembled.  If the center of the disk is not aligned to the shaft of the spindle motor, the servo information may be recorded not on a region around the shaft of the spindle motor, but a region centered on the disk.  As is shown in FIGS.  2A and 2B, a trajectory of the head moving along the servo-information-recorded positions is different from an actual trajectory of the disk rotating.  As a result, hard disk drives have different following-prohibited regions of their own disks. 
In the conventional hard disk drives, the same positions on the disks of the different hard disk drives are determined to be the logical track zero position irrespective of assembled states of the disk drives.  Therefore, there is a problem in that the logical track zero position may exist on the following-prohibited region.  In addition, even if the logical track zero position exists on a following-allowable region, the hard disk drive having defects on a system region may be treated as a defective hard disk drive.