The Thumbnail cache will have nothing to do with your drives sometimes showing up and sometimes not. That could be a possible hardware failure on your drive(s). The main way to check this is to click on My Computer, right click on each drive, go to Tools, and Check Disk.
However, the fact that no DVD drives are showing up could mean this is a larger issue with the system itself. Are you running any antivirus at all? Have you installed anything relatively recently?