Patent ID: 7319567

Claim:
A magnetic disk drive comprising: a magnetic disk having a plurality of concentric tracks on each of which N servo sectors containing servo information are disposed alternately with data sections; a spindle motor which bears and rotates the magnetic disk; a magnetic head which writes and reads data to and from the magnetic disk; means for moving the magnetic head in a radial direction of the magnetic disk; and a decoding circuit which decodes the servo information read by the magnetic head; wherein each servo sector has a servo sector number section where k bits of servo sector number information, fewer than the bits required to express the servo sector number itself, are written; and wherein said magnetic disk drive further comprises: storage means for storing the k-bit servo sector number information which has been read from each of m successive servo sectors and decoded by the decoding circuit; m different servo sector number determinators which determines the servo sector numbers of servo sectors from the m pieces of k-bit servo sector number information (where, m(2 m×k −1)≦N); and selecting means for selecting one of the m different servo sector number determinators.