DISCLAIMER: Your Linux box won’t normally prevent you unmounting something without a reason. Run these commands at your own risk… to data integrity.

If you’re attempting to unmount a volume (such as /mnt), and it refuses…

# umount /mnt

==> umount: /mnt: device is busy.
==>        (In some cases useful info about processes that use
==>         the device is found by lsof(8) or fuser(1))

…there’s a chance a process is still accessing it. You can use lsof to see:

bash  14115  root  cwd  DIR  202,16  4096  2  /mnt

Great, in this case the only thing using it is our shell, and we’re not even in the directory. In that case, time to force:

# umount -f /mnt

==> umount2: Device or resource busy
==> umount: /mnt: device is busy.
==>         (In some cases useful info about processes that use
==>          the device is found by lsof(8) or fuser(1))
==> umount2: Device or resource busy

For more persistent errors, its time to kill the process using it with fsuer.

# fuser -km /mnt
==> /mnt:                14115c
# umount /mnt

Then the volume should unmount. Now would also be a good time to re–read the disclaimer at the beginning of this post.