I probably won't get around to using it, but a couple of years ago I had a disk get its Host Protected Area set (by a maliciously well-intentioned external drive enclosure), and after I couldn't fix it, I went to my friend the late Hugh Daniel, and he and I spent a long evening trying to get the Linux HPA tools to work, rebuilt Linux kernels a couple of times, consumed lots of pizza, and only succeeded in making the HPA bigger, never smaller. The tools just weren't good enough, and the documentation on HPA was deliberately unavailable. Fixing a 500 GB PATA drive is probably not worth it at this point, but it'd be a fun hack to do in memory of Hugh.
For those of you who've never met HPA before, it's a different set of BIOS interrupts for talking to disk drives which let you allocate space that Windows can't touch, so you can do things like hide a system-restore partition on the drive, or turn a 200 GB drive into a 128 GB drive (so an old computer that can't read LBA can at least use the 128 GB it understands), or turn a 250 GB drive with bad blocks into a 200 GB drive without them (so you can sell the stuff that didn't pass quality control.) In my case, I had an old Maxtor 200GB external USB drive that was failing from too many bad blocks, so I replaced the disk with a new 500GB one. The drive enclosure didn't recognize the disk, so it wrote a 300 GB HPA to knock it down to the same 200 GB size of the original one.