[dnanian]

OK, Pat. It sounds like the external drive failed and "fell off the bus", but this could be due to an AntiVirus program, too.

If you're using Intego's VirusBarrier, you should simply restart your machine and the space will return. Make sure to disable VirusBarrier, and try your backup again.

If not, eject the external drive if still attached, and use Finder's "Go To Folder" command to open the "/Volumes" folder. Look in there for a folder named the same as the external drive. If there, delete that and empty your trash, and the space will return.

Now, though, you need to determine why the drive failed in such a spectacular way. Do you have anything else attached to FireWire?