Weird problem, simple solution. I have an external Maxtor backup disk connected to my Macbook Pro, and I cannot eject it. I get the error:

"DRIVENAME is in use and could not be ejected."

First, I try to use the lsof command to figure out what program still had open files on the drive. Unfortunately, I get no output, meaning no files are open on the drive.

Next I resorted to the Mac OSX hdiutil command. Using sudo hdiutil unmount reported that it was already unmounted.

Finally:

sudo hdiutil eject -force diskname

And the drive immediately disappeared from the Desktop. Hallelujah!