DU "can't unmount disk"?

[Onamuji]

If you're erasing it then you don't have to treat the contents with special care. Stuff that might corrupt the directory structure is fair game because you'll be making a nice clean new directory-structure.

Eject from the Finder and when it tells you that the drive is busy and asks if you want to force-eject it then opt to do so.

Then you'll be able to erase it with the DU.