@BobRoss I'll admit to joining this thread late and skimming it. So, hopefully my attempt to help won't actually hurt. If you were able to run a level 4 scan on the suspect hdd, that means two things. 1) The computer and bios that you ran the scan on can see and talk to the HDD. 2) If it passed a level 4 scan, that means all the sectors were read / inverted / written / read / inverted / written properly. It DOESN'T mean that all the data on the HDD is "good", IE what windows expects it to be. So, on THAT PC, the one where you ran the Spinrite scan, can you boot Windows, attach the questionable drive via the drive bay, which I presume is USB, and see the drive in Windows Explorer. If you can, right click on the drive in Explorer, go to properties, then tools, then Error Checking, Check Now. You may have to enter an admin password. UNCHECK the box that says automatically fix errors. This will run a CHKDISK on the unit, but NOT make any changes to it. If the file system checks out, you may have lost the boot sector, which would need repair in that case. At least, this is the way things work on Windows 7. If you can do these tests, let us know what happens.
May your bits be stable and your interfaces be fast.

Ron