$ sudo -i umount /extraumount: /extra: device is busy.
(In some cases useful info about processes that use
the device is found by lsof(8) or fuser(1))
$ sudo -i fuser -km /extra
$ sudo -i umount /extra
-- You can also try umount command with –l option:
# umount -l /mnt
-l : Also known as Lazy unmount. Detach the filesystem from the filesystem hierarchy now, and cleanup all references to the filesystem as soon as it is not busy anymore. This option works with kernel version 2.4.11+ and above only.
Caution: Using these commands or option can cause data loss for open files; programs which access files after the file system has been unmounted will get an error.