http://answers.microsoft.com/en-us/windows/forum/windows_7-hardware/my-external-hard-drives-disappear-and-are-not/4fefdd4d-20ef-4885-ba38-8ea435d6799a

http://support.microsoft.com/kb/976972

Things to try:

• Control Panel → Power Options → Change Plan Settings → Change Advanced Settings →
  • USB Settings → USB Selective Suspend → Disabled
  • Hard Disk → Turn Off Hard Disk → Never
• Change the USB cable for a shielded cable of better quality
• If you have an nVidia chipset, install the hotfix
  • http://support.microsoft.com/kb/976972