Hummm... no luck yet. HDInstTools gets a little further than HDToolbox and was able to correctly identify one of the drives (the other drive doesn't show up at all in either application), but no partition table shows up and attempting to make any changes to the drive at all results in an error message.
I also remembered I had an ancient IDE-to-USB external chassis, threw one of the drives in there but Windows was unable to detect it (in the Computer Management -> Disk Management console), either. The interface itself showed up, but with no drive attached.
As a last option I downloaded WinUAE, but I'm so used to doing things on a real Amiga I have no idea how to try to mount a hard drive in that application, LOL.

Regardless, I suspect if the PC can't detect the drive, WinUAE wouldn't see anything either.
May try replacing the controller boards, or poking around at a few other solutions. Not a high priority. It's waited this long, a little longer won't kill me, haha.
