Hello, & Welcome to Tom's Hardware!
What is the Make/Model# of your external HDD, and is it connected by a USB connector? Are you connecting it to a Win-7, Vista, or Win-XP computer?
Could you please connect it to your desktop computer, go to Disk Management (right click on my computer, manage, computer management, lower left corner click on Disk Management).
Up load a screenshot of the Disk Management dialog box, especially the lower graphical area, to post on your response. That way someone can tell exactly what the problem is and how to best helop you.