Hard disk failure of some sort - files still there, filesystem gone!

[Inconsequential]

Disktest gives this:

check_FAT: Unusual media descriptor (0xf0!=0xf8)
Warning: Incorrect number of heads/cylinder 16 (FAT) != 1 (HD)
Warning: Incorrect number of sectors per track 32 (FAT) != 1 (HD)
EFI System 40 409639 409600 [EFI]
Mac HFS 409640 3906766983 3906357344

What's wrong with it, how do I fix it?

All the files are there, I just can't get at them through OS X!

It's an advanced format drive so maybe something isn't playing ball?

 

[davidlv]

Disktest gives this:
check_FAT: Unusual media descriptor (0xf0!=0xf8)
Warning: Incorrect number of heads/cylinder 16 (FAT) != 1 (HD)
Warning: Incorrect number of sectors per track 32 (FAT) != 1 (HD)
EFI System 40 409639 409600 [EFI]
Mac HFS 409640 3906766983 3906357344
What's wrong with it, how do I fix it?
All the files are there, I just can't get at them through OS X!
It's an advanced format drive so maybe something isn't playing ball?

Did you try booting up from the original OS X Install DVD? (c key or Option down at startup) Run Disk Utility from the Utilities Menu after selecting a language.
If Disk Utility can not repair the errors, you should try Disk Warrior which requires a bootable DVD (Alsoft's DVD will boot all but the latest Macs, check the web site for full info) or a bootable external HD with Disk Warrior installed.
If Disk Warrior can not fix it, it is likely there is a hardware issue and you will have replace the HD.