Disk drive comprising asynchronous/synchronous gain control for fault tolerant detection of servo sync mark after head switch

A disk drive comprising:

• (a) a plurality of disk surfaces each having a plurality of tracks, each track comprising a plurality of data sectors and a plurality of embedded servo sectors, each embedded servo sector comprising a servo sync mark for synchronizing to the embedded servo sector;
• (b) a plurality of heads actuated radially over respective disk surfaces;
• (c) head selection circuitry for selecting one of the heads to perform a read or write operation;
• (d) a gain control system implementing an asynchronous gain control algorithm and a synchronous gain control algorithm for controlling a gain of a read signal emanating from the selected head;
• and
• (e) a synchronous sync mark detector for synchronously detecting the servo sync mark from the read signal, wherein:
• when the disk drive is synchronized to the embedded servo sectors of a first disk surface corresponding to the selected head, the synchronous sync mark detector is enabled relative to a detection window for detecting the servo sync marks recorded on the first disk surface;
• when the head selection circuitry selects another one of the heads corresponding to a second disk surface, the detection window is opened early to detect a servo sync mark recorded on the second disk surface;
• the asynchronous gain control algorithm prevents the gain control system from diverging due to the newly selected head reading an area of the second disk surface preceding the servo sync mark;
• and
• the synchronous gain control algorithm for maintaining a proper gain for the read signal while the newly selected head reads the servo sync mark.