How do I backup my USB Thumbdrive?

I have Vista Business. Trying to use the Vista Backup and Restore Center.
Using the option to "Back up files".

I want to backup just my 4GB USB Thumbdrive to another USB drive that I use
for storing backups.

I am not able to select the 4GB USB Thumbdrive as a source drive. It
doesn't show up. All other drives show up as an available source, including
the USB drive that I am trying to backup to.

Any idea why a USB Thumbdrive is not available as the source drive? Is
there anyway to force the software to show the drive?

Thanks,
Butch