Due to OS upgrades, sometimes, a hard drive that worked fine previously fails to show up in (My) Computer.
This happened to me once when I upgraded a computer from Windows Vista to Windows 7. The external hard drive was powered up, and detected by the system (proven by the fact that it shows up as disk drive in Device Manager), but it did not show up in My Computer.
The hard drive was a relatively new model and perfectly supported by both Windows Vista and Windows 7.
- This might be caused by driver issues, so with the external USB hard drive plugged in, go to Device Manager (right-click My Computer > Manage > Device Manager).
- Expand the USB list if it is a portable hard drive, or Disk Drive if it is a 3.5″ hard drive.
- Find the device, and right-click > Uninstall it.
- Turn off and on the hard drive, and let Windows install the required drivers.
That should fix the issue.