Disk drive adjusting erasure window based on a number of non-read data sectors during a retry operation

1. A disk drive comprising:

a disk comprising a plurality of data tracks, wherein each data track comprises a plurality of data sectors;

a head actuated radially over the disk; and

control circuitry operable to:

read one of the data sectors from a target data track on the disk;

store the data sector in a buffer;

first decode the data sector with an error correction code (ECC) decoder;

when the first decode fails:

adjust an erasure window based on a number of non-read data sectors recorded in the target data track, wherein the erasure window comprises a length spanning at least one ECC symbol in the data sector;

read the data sector from the buffer; and

second decode the data sector with the ECC decoder using the erasure window.

2. The disk drive as recited in claim 1 , wherein when the second decode fails the control circuitry is further operable to:

step the erasure window by a step size equal to at least one ECC symbol in the data sector;

read the data sector from the buffer; and

third decode the data sector with the ECC decoder using the erasure window.

3. The disk drive as recited in claim 2 , wherein adjusting the erasure window comprises adjusting at least one of the length of the erasure window and the step size.

4. The disk drive as recited in claim 2 , wherein the control circuitry is further operable to repeatedly step the erasure window by the step size and decode the data sector using the ECC decoder during a retry operation prior to the head reaching the data sector recorded in the target track.

5. The disk drive as recited in claim 4 , wherein the control circuitry is further operable to step the erasure window over the entire data sector prior to the head reaching the data sector recorded in the target data track.

6. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to:

initialize an offset for the erasure window;

step the erasure window starting at the offset during a first retry operation;

adjust the offset for the erasure window after the first retry operation; and

step the erasure window starting at the adjusted offset during a second retry operation.

7. The disk drive as recited in claim 1 , wherein:

the disk comprises a plurality of zones;

each zone comprises a plurality of the data tracks;

a data rate of a first zone is less than a data rate of a second zone; and

the control circuitry adjusts the erasure window relative to the zone comprising the target data track.

8. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to:

generate a disk locked clock synchronized to a data rate of the data sector recorded in the target data track; and

generate a fixed rate clock;

wherein:

the first decode of the data sector with the ECC decoder uses the disk locked clock; and

the second decode the data sector with the ECC decoder uses the fixed rate clock.

9. The disk drive as recited in claim 8 , wherein a frequency of the fixed rate clock is higher than a frequency of the disk locked clock.

10. The disk drive as recited in claim 1 , wherein the control circuitry is further operable to second decode the data sector while the head is passing over the non-read data sectors recorded in the target data track.

11. A method of operating a disk drive, the disk drive comprising a disk comprising a plurality of data tracks, wherein each data track comprises a plurality of data sectors, and a head actuated radially over the disk, the method comprising:

reading one of the data sectors from a target data track on the disk;

storing the data sector in a buffer;

first decoding the data sector with an error correction code (ECC) decoder;

when the first decoding fails:

adjusting an erasure window based on a number of non-read data sectors recorded in the target data track, wherein the erasure window comprises a length spanning at least one ECC symbol in the data sector;

reading the data sector from the buffer; and

second decoding the data sector with the ECC decoder using the erasure window.

12. The method as recited in claim 11 , wherein when the second decoding fails further comprising:

stepping the erasure window by a step size equal to at least one ECC symbol in the data sector;

reading the data sector from the buffer; and

third decoding the data sector with the ECC decoder using the erasure window.

13. The method as recited in claim 12 , wherein adjusting the erasure window comprises adjusting at least one of the length of the erasure window and the step size.

14. The method as recited in claim 12 , further comprising repeatedly stepping the erasure window by the step size and decoding the data sector using the ECC decoder during a retry operation prior to the head reaching the data sector recorded in the target track.

15. The method as recited in claim 14 , further comprising stepping the erasure window over the entire data sector prior to the head reaching the data sector recorded in the target data track.

16. The method as recited in claim 11 , further comprising:

initializing an offset for the erasure window;

stepping the erasure window starting at the offset during a first retry operation;

adjusting the offset for the erasure window after the first retry operation; and

stepping the erasure window starting at the adjusted offset during a second retry operation.

17. The method as recited in claim 11 , wherein:

the disk comprises a plurality of zones;

each zone comprises a plurality of the data tracks;

a data rate of a first zone is less than a data rate of a second zone; and

the method further comprises adjusting the erasure window relative to the zone comprising the target data track.

18. The method as recited in claim 11 , further comprising:

generating a disk locked clock synchronized to a data rate of the data sector recorded in the target data track; and

generating a fixed rate clock;

wherein:

the first decoding of the data sector with the ECC decoder uses the disk locked clock; and

the second decoding of the data sector with the ECC decoder uses the fixed rate clock.

19. The method as recited in claim 18 , wherein a frequency of the fixed rate clock is higher than a frequency of the disk locked clock.

20. The method as recited in claim 11 , further comprising second decoding the data sector while the head is passing over the non-read data sectors recorded in the target data track.