If you get a "device is busy" error while trying to umount a drive, make sure that no other user is in that directory. Make sure you didnt su yourself and the login-user is sitting in the directory you want to umount... make sure no one ... well, you know the hundreds of reasons why you couldn't umount.
[Solution]
If you want to know exactly why you cant umount, for example, /mnt/mymounteddrive - just use this:
lsof | grep /mnt/mymounteddrive
You will get all files and users that access that mount point atm - as well as the pids of their processes. You know how to use the kill <pid> command, right?
