Have you ever had this problem of unmounting your CDROM that comes with the error that the device is busy or whatever?!
Now, what you want is to force it to unmount so you do a "sudo umount -f /media/cdrom0".
It won't work. Try to do a lazy unmount instead:
$sudo umount -l /media/cdrom0
and most of the time, it will work. :) It may also work on other devices.
P.S.: If everything else fails, press the CPU OFF button then pull out your CD during boot-up! Now that's lazy unmount! :)