If it is okay in the car, you might want to check to see if your AC power supply / wall wart is dying. Maybe it is not supplying enough power to keep the drives spinning...

Another thought, if you have one of Mark's docks, is to try it in there - that uses the DC power channel.

Edit: Note - this assume that it was working in the car. It was in one of your posts, but not in the first post. You might also just want to check to see if the hard drive cable worked loose from the connector due to use or vibration.