A method of writing product servo sectors to a disk of a disk drive is disclosed. An external spiral servo writer writes a plurality of spiral tracks to the disk by actuating in a closed loop system a head positioning pin inserted into the disk drive. Each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark. The high frequency signal and sync marks in the spiral tracks are demodulated to maintain synchronization of a servo write clock as well as maintain the head along a circular target path while writing product servo sectors to the disk.