Sounds like a SSD drive connected via SATA. You've already tried swapping SATA cable, did you already try plugging into a different SATA port? If the SSD doesn't show up at all then it's probably gone at this point.. hopefully you have backups.

You could also try installing the SSD drive into a good external USB case/adapter but I doubt that'll yield different results.

Not sure about all the other stuff you're doing, unless this issue occurred right after you updated or reset your BIOS there should be no reason to mess with BIOS downgrades, RAM speeds, SATA settings, etc.

Sounds like your drive is just dying, barring any other power issues in your system.

You sure your voltages are good after increasing your memory clock speed? Perhaps your North Bridge is getting reset because of power issues, and your drive is losing connectivity.

I'd test it from another machine, or perhaps an external reader if you have one, but more than likely, your drive is just dying.