Several weeks ago I junked an iMac G5 whose USB ports had somehow gotten fried. But before I recycled it at the city's e-waste center I pulled out the SATA hard drive, figuring I could use it elsewhere.
I wanted to use it as a time machine disk, inside a new Mac Pro. I was very impressed with how easy it was to install the disk--the Mac Pro is prettier on the inside, from a form follows function perspective, than the outside.
However, I found that the disk would unmount every time the computer slept. Upon waking it was simply missing. Not even System Profiler could see it. Restarting the computer would cause the drive to reappear. Clearly this wasn't going to work as a backup strategy, the time between restarts on my computers is measured in weeks.
It turns out the problem is that the Mac Pro requires a SATA 3 GB drive. (That's the data transfer speed, I believe.) A quick visit to Fry's and I had a 1.5 TB SATA 3 GB to replace the one I pulled, and it's working perfectly. (And it only cost $140, amazing!)
For now the drive I pulled from the iMac is sitting unused. I'll probably get an external enclosure for it at some point. But if you've got a Mac Pro, and finding that a drive disappears when the computer sleeps, check to see if the drive is up-to-spec. If it's an older drive, chances are its not.