I haven't tried this on an external drive before but I assume it would work... When I need to format a hard drive, I use a program called Partition Logic, it's free and it does a good job. You need to burn it to an ISO image on a CD though and then just boot from the CD and it will come up with a GUI. It's pretty simple to use and I like it. You can get it at http://partitionlogic.org.uk/index.html