What EXACTLY do you mean?
Do you mean that the external drive does not show up in "My Computer" or Windows Explorer?
Do you mean that the drive shows up but you get an error message just by selecting the drive?
Or do you mean that the drive shows up and allows you to see the directories but does not allow you to access the directories or files?
If the last, discover the joys of TAKING OWNERSHIP -
If not that last, then we definitely need more specifics.