The mysteriously missing hard drive
Suddenly my wife's laptop wouldn't boot. The internal drive disappeared. It wasn't corrupted or failed, as far as the system went, it didn't exist.
I found that if I took out the drive and installed it in an external enclosure, it worked just fine. If I installed another drive internally, it wasn't recognized either.
I ordered a replacement cable from iFixit, hoping that it was the issue, even though the cable showed no damage. I installed the cable easily and everything was back working in 15 minutes!
Even though the cable doesn't show any damage, and nothing seems to be directly the cause for the missing hard drive, it is probably the cable.