Hmmm that error message is typically what you get when you don't use "run as administrator". Strange.
If this is a work computer, then I can understand it, if not, still scratching my head.
An alternate route is to do it through the GUI (My Computer, right-click on the external drive, Properties, Tools tab, Error checking - make sure both boxes are checked.). See if you can run it that way.
Last resort, break out that hammer.