The "umount" command has a "-f" option that's not documented in the manpage. % umount --help Usage: umount [-hV] umount -a [-f] [-r] [-n] [-v] [-t vfstypes] umount [-f] [-r] [-n] [-v] special | node...
I have verified this to be true.
The -f is not documented because it's not available on all platforms since the functionality of the flag depends on the existence of a system call.