Whenever I have seen something like this suddenly occur when it was working fine before, it is usually a loose cable, either power cable or interface. Sometimes you can access via DOS, but once in a 32bit system thats when it gets lost. I cant explain why but thats the way it has worked for me. Its been things like one pin in the molex cable loose, or the interface cable partially pulled loose. Or possibly the one thing you dont want to hear.. a drive going bad. I would check power and interface cables for sure if there have been no major changes to software/hardware/bios settings.