Abstract: A disk drive system includes a magnetic disk apparatus which accurately positions read/write heads for reading data from and writing data on tracks on the data surface of a rotary disk on a predetermined track on the disk on the basis of servo data written in the servo area formed on the data surface, and a disk controller which controls the mode of operation of the disk apparatus. Index signals respectively indicating the start and end of the servo area are generated on the basis of the result of detection of the angular phase of the disk. The mode of operation of the magnetic disk apparatus is determined on the basis of a write control signal provided by the disk controller. The index signals respectively indicating the start and end of the servo area are sent to the disk controller when the mode of operation is track format write mode, while the index signal indicating the end of the servo area is sent to the disk controller when the mode of operation is other than the track format write mode.