Servo Area Corruption Western Digital

Western Digital P-List corruption causing drive failure

Background: Failures on WD hard disks can sometimes be attributed to a corruption of service data located in the firmware zone of the hard disk. This corruption often involves a table commonly known as the P-List and in many cases, it is impossible to read the original contents of the P-List once the drive has failed.

What is the P-List?: The P-List is a growing table of defects that the drive uses to avoid bad sectors that have developed since its factory formatting, the drive marks a sector as bad and then allocates a reserve sector from a special area for the drive to use so that the LBA of the drive does not change.
Operation: At start up, the WD hard disk loads the service data from its firmware zone into RAM and calculates the checksums of certain critical modules before allowing the drive to report on readiness. One of these modules is the P-List. When one of these drives fail with a P-List fault it will:

• not report readiness
• will be recognised by the PC BIOS by its factory alias
• refuse to allow access to user data

Possible repair and data recovery: Despite P-List inaccessibility, successful recovery of the drive and the data on it is often possible as a complete copy of the defects stored within the P-List is also located in the Pivot Defects table, the same drive table that is actually used by the drive when loading the defects and translating the Logical Block Addressing (LBA) of the drive at start-up.

The only reason that the drive will not start with a corrupt P-List is that it does not have the ability to repair the P-List itself and thus has nowhere to store any new defects that may develop during use. Because of this unique feature of the WD hard disk it is almost always possible to perform a near 90% data recovery of the data from the drive by using a multi-stage repair process.

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