umount - unmount a mounted file system
umount /dev/fd1 # Unmount diskette 1
A mounted file system is unmounted after the cache has been flushed to
disk. A diskette should never be removed while it is mounted. If this
happens, and is discovered before another diskette is inserted, the
original one can be replaced without harm. Attempts to unmount a file
system holding working directories or open files will be rejected with a
'device busy' message.