Thanks for your replies. I have just now tried the drives, one by one in a different computer. They still show as working fine, but can't see disks when inserted. They are on the secondary IDE channel, but that shouldn't matter. Lots of CDs work as a slave on the main channel. I hadn't considered dust on the lens. That happening to three drives at once seems unlikely, especially since the system was assembled in June, and has been vacummed out at least once since then, when I installed the DVD ROM. I was thinking they might have been zapped by the power supply, as it was a new thing as of last June. I put the computer in a new case then, and used the supply that came with it. Does this sound like a problem than could be caused by power fluctuations?