HPA: Host Protected Area. The HPA is an area at the end of an IDE (ATA) hard drive marked by a disk drive manufacturer or OEM disk distributor as not being user accessible. Conventional uses include isolating configuration data, marking off spare tracks, storing restorable copies of the Operating System, storing diagnostic software, and storing error detection records. HPAs can be password protected. HPAs are not part of the normal file system. They are not usually reflected in the partition table although nothing prevents them from being there if an HPA address is (re)set on a partitioned drive. The disk sectors will not be accessible until the HPA start address is reset again, and might well cause trouble.

For the most part HPAs are invisible to software that is not HPA aware although their presence can possibly be inferred by comparing total sectors claimed by the disk manufacturer to total sectors reported by the operating system. Some versions of Linux appear to report the number of HPAs (one or zero with current HPA implementation) along with other disk information during boot.

The capability to support HPAs was added in Version 4.0 of the ATA specification. HPAs can not be set up on drives built before roughly 1999 (give or take a bit). It is reported that some older drives may misbehave if asked about the presence of HPAs. They shouldn't, but testing for proper (non-)handling of commands that are not yet defined isn't always perfect. HPA is only supported on IDE, not SCSI. At least one HPA sensitive program (Declasfy) warns that trying to release the nonexistent HPA on a SCSI drive may "result in damage"

HPA settings can be temporary (until the next computer boot) or permanent.

Most disk copy programs do not copy HPAs, but HPA aware disk copy programs do exist. There are also programs that can make HPAs user accessible. The effect of altering or releasing an HPA will be to change the perceived disk drive size, which has a certain potential for confusing programs that assume that disk drive sizes do not change dynamically

Many forensic disk analysis tools handle HPAs in some fashion since information could easily be hidden from normal access by setting up or expanding an HPA.

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