Servo writing a disk drive by overwriting a harmonic frequency fill pattern in the servo burst area

A disk drive comprising:

• (a) a disk comprising a plurality of tracks, wherein:
• each track comprises a plurality of data sectors and a plurality of servo sectors;
• each servo sector comprises a plurality of servo bursts written using perpendicular magnetic recording;
• each servo burst is written at a servo burst frequency;
• a fill pattern is written between the servo bursts using perpendicular magnetic recording at a frequency substantially equal to an even harmonic of the servo burst frequency;
• and
• a phase shift occurs in a read signal at the transition between the servo burst and the fill pattern;
• (b) a head actuated over the disk for generating the read signal;
• (c) a sampling device for sampling the read signal to generate a sequence of read signal sample values;
• and
• (d) control circuitry for demodulating the servo bursts from the sequence of read signal sample values by computing a single-point Discrete Fourier Transform (DFT) at the servo burst frequency.