For anyone that has CIFS Shares mounted in a Linux machine, sooner or later you will get the an error saying that you cannot unmount a drive because it is busy.
So how do we find out what process is keeping that drive busy?
Simple. Just type on the console or X terminal:
lsof +D /path/to/mountpoint
This command returns the command and process ID of any tasks currently accessing the mount point, and you kill the process.