The only time I have ever had this problem it was a power issue. A power supply tester that tests individual power feeds (you can find them on NewEgg.com for under $20) or a simple multimeter will tell you if you're getting the proper voltage. That being said, it could be the hard drive controller. Putting the drive into the kids computer to see if it would work would be a good idea to try and would not harm the registry. Any registry entries added would be removed when you took the drive back out. Check the power supply first and work from there.
I have 2 120gb HDDs in my HP a250n, this had been working fine for several months. About one month ago the D: (non-system) disc disappeared for 2 or 3 days, then sort of came back from vacation. Last week the same drive took off again for 2 days, I opened the box and checked all the connections restarted and there it was back again. This AM it was gone again, restarted and is back.
I have the SMART thing that shows up on Belarc report which I ran last night and both HDD show their health as Healthy. This always seems to happen overnight with computer left on. I suppose the obvious solution is turn it off and sometimes do with no prob. Both drives work fine in a sort of binary fashion. If there it works, if not who knows? My 425 watt psu is 2 mos old. Could it be the wires to the HDD? the motherboard? or something with the HDD itself. I can put up with this or put that drive in the kids puter to see if it shows up, but the registry will be all messed up there and have to do without that I guess. This is far from the end of the world but I would like to get some suggestions.