Estimating a writer/reader gap in a disk drive by measuring write/read times relative to a sync mark

A method of estimating a writer/reader gap in a disk drive, the disk drive comprising a disk having a first preamble and a first sync mark, and a head actuated over the disk, wherein the head comprises a writer separated from a reader by the writer/reader gap, the method comprising the steps of:

• (a) measuring a write delay during a first revolution of the disk by:
• reading the first preamble to synchronize cycling of a clock;
• reading and detecting the first sync mark;
• waiting a predetermined number of the clock cycles;
• writing a second preamble and a second sync mark on the disk;
• and
• counting a number of the clock cycles relative to when the first sync mark is detected and when the second sync mark is written;
• (b) measuring a read delay during a second revolution of the disk by:
• reading the first preamble to synchronize cycling of the clock;
• reading and detecting the first sync mark;
• waiting a predetermined number of the clock cycles;
• reading the second preamble to synchronize cycling of the clock;
• reading and detecting the second sync mark;
• and
• counting a number of the clock cycles relative to when the first sync mark is detected and when the second sync mark is detected;
• and
• (c) estimating the writer/reader gap in response to the measured write delay and the measured read delay.