BartPE is a stripped down version of XP... my point still stands
anyway, AHCI mode can cause misery when you're using a non-intel controller usually, and even intel controllers may not be immune to problems... so for the sake of eliminating problems, put it back into IDE/legacy mode

as mentioned, I did try fiddling with the BIOS settings... I remembered the trick of disabling AHCI from the pre-SP3 days, but this time it didn't do me any good, neither with BartPE nor with UBCD4win.

though I'm happy to say I've rescued most of my important stuff. Parted Magic did the trick: finally got it to boot up after I blacklisted some problematic video driver, and it saw both SATA drives just fine. As for the bad disk, 95% of the crucial data was on the non-OS partition, and luckily I managed to grab it all without a single error.
The filesystem on the OS partition (C:) is fritzed though. Directories showing up empty, most files reported as 0 bytes, virtually nothing that can be copied correctly. Will try some more diagnostics, but this still stinks of physical failure somewhere... so I guess this drive is destined for the dumpster.

On the upside, it's an excuse to be dragged kicking and screaming into the windows 7 age. ;)

Since it's a Seagate drive and you got the important data out...just out of curiosity, why not run Seatools on it (free from Seagate, use the bootable version) See if it's really screwed and bin material.

update the firmware then run seatools and use the long test so that it'll remap any bad sectors it could... don't put your faith in that drive though... when it starts dying, it's a downward spiral, just like what I experienced with my barracuda 7200.12

I wouldn't take that chance... I'd toss it in the trash!