I had something similar a while ago on an XP machine. In the end it came down to the drive letter that the External drive was using had been mapped by something else, and as a result of the conflict the external drive lost out and never appeared.
I can't remember exactly where I deleted the unwanted mapping , but look for unused drives in the F-H range ( your PC may be different ) and remove them It may allow the HDD to by properly found
Good luck
D.