Adjusting track density by changing slope of spiral tracks used to servo write a disk drive

1. A method of writing product servo sectors to a disk of a disk drive to define a plurality of data tracks, the disk drive comprising control circuitry and a head disk assembly (HDA) comprising the disk, an actuator arm, a head coupled to a distal end of the actuator arm, and a voice coil motor for rotating the actuator arm about a pivot to position the head radially over the disk, the method comprising the steps of:

(a) writing a first plurality of spiral tracks having a first slope to a first disk surface, wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark;

(b) writing a second plurality of spiral tracks having a second slope substantially steeper than the first slope to a second disk surface, wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark;

(c) using the head internal to the disk drive to read the spiral tracks to generate a read signal;

(d) processing the read signal to detect the sync marks in the spiral tracks to synchronize a servo write clock;

(e) processing the read signal to demodulate the high frequency signal in the spiral tracks to generate a position error signal used to maintain the head internal to the disk drive along a first target circular path; and

(f) using the head internal to the disk drive and the servo write clock to write product servo sectors along the first target circular path;

wherein the steeper slope of the second plurality of spiral tracks causes a track density of the data tracks to be lower on the second disk surface compared to the track density of the data tracks on the first disk surface.

2. The method as recited in claim 1 , wherein:

(a) the head internal to the disk drive is used to write the spiral tracks to the disk;

(b) the actuator arm is rotated about a pivot to move the head radially across the disk while writing the spiral tracks; and

(c) the actuator arm is moved at an angular velocity selected in response to the target track density.

3. A disk drive comprising:

(a) a first disk surface comprising a first plurality of spiral tracks having a first slope selected in response to a target track density, wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark;

(b) a first head for interfacing with the first disk surface;

(c) a second disk surface comprising a second plurality of spiral tracks having a second slope substantially steeper than the first slope, wherein each spiral track comprises a high frequency signal interrupted at a predetermined interval by a sync mark;

(d) a second head for interfacing with the second disk surface; and

(e) control circuitry for writing product servo sectors to define a plurality of data tracks on the first disk surface and the second disk surface by:

reading the spiral tracks to generate a read signal;

processing the read signal to detect the sync marks in the spiral tracks to synchronize a servo write clock;

processing the read signal to demodulate the high frequency signal in the spiral tracks to generate a position error signal; and

using the servo write clock and position error signal to write product servo sectors;

wherein the steeper slope of the second plurality of spiral tracks causes a track density of the data tracks to be lower on the second disk surface compared to the track density of the data tracks on the first disk surface.

4. The disk drive as recited in claim 3 , wherein:

(a) the first and second head internal to the disk drive are used to write the spiral tracks to the disk;

(b) the first head is connected to a distal end of a first actuator arm;

(c) the second head is connected to a distal end of a second actuator arm;

(d) the actuator arms are rotated about a pivot to move the first and second head radially across the first and second disk surface while writing the spiral tracks; and

(e) the first actuator arm is moved at a first angular velocity while writing the first plurality of spiral tracks to the first disk surface; and

(f) the second actuator is moved at a second angular velocity while writing the second plurality of spiral tracks to the second disk surface, wherein the second angular velocity is substantially greater than the first angular velocity.