Basically, if you, as a regular user, cannot unmount a device, you can usually just go to root shell (sudo -i), see where the thing is mounted (just use mount) and then unmount it (umount /media/blah/floop). If it says the device is still busy, as root you can remount it as read-only and then unmount it (seeking documentation left as an exericise to the reader, as I'm sitting at a Windows system now).
There's also lsof ("list open files") tool, which can be used to see which exact programs are using files on that device, which you can then use to kill those sneaky processes (ps, kill).