Sometimes we might get the message "device busy" on trying umount.
This is because some processes are still accessing those device and we need to stop them before umount.
The following command can be used to get the list of processed those using a specific mount point.
#lsof | grep <mount-point>
Where mount-point need to be replaced with the path to which the device is mounted.
We need to terminate these processes and then umount will be a success.