A method of estimating a writer / reader gap in a disk drive is disclosed. During a first revolution of the disk, a
clock is synchronized to a first
preamble and used to measure a write
delay to write a second
sync mark to the disk relative to a first
sync mark. During a second revolution of the disk, the
clock is again synchronized to the first
preamble and used to measure a read
delay required to detect the second
sync mark relative to the first sync mark. The writer / reader gap is then estimated in response to the measured write
delay and the measured read delay, for example, by computing a difference between the measured write delay and the measured read delay.