Hi Beth, & Welcome to Tom's Hardware!
What is the make and model # of the ext HDD? What OS are you running on your desktop/laptop computer?
The best thing to do for us to see exactly what is going on, is to connect the external HDD to your computer, boot it up, then go to Disk Management (right click My Computer, Manage, Disk Management in lower left)
Upload a screenshot of the Disk Management dialog box (photobucket or imageshack) so we can see the Disk Info and Volume Info, and what might be the problem.