Hardware

Google is your friend. Fixes abound all over the internet. It took 5 seconds to find the following:

First, read this article to explain what the error means. Note, it refers to Win2K, but the general information is the same.

http://support.microsoft.com/default.aspx?scid=kb;en-us;141117

Next, now that you understand what's wrong, I sure hope you've got a good backup of all your files, because the following is what you need to do. If your 'important' data is damaged, you'll be up a creek without backups. So.........

Assuming you've got WinXP, click START, click RUN and type "cmd" without the quotes. You'll be in command window - then type chkdsk [drive:] /F (such as - chkdsk e: /F) which should perform a repair of the drive. (NOTE: replace the 'e:' in the example with the drive letter of your external drive.) Wait for it to finish and close the command window once you've read the report of what it found.

If you're not running WinXP, how about telling us what you really have?

If the data is valuable you should try this but it's expensive.

Ontrack Data Recovery

http://www.ontrackdatarecovery.com

Lucky I looked before I posted as Thummy has provided a good tip with chkdsk.