How to unmount a dead smbfs file system.
Little known fact (to me, at least): if you have a mounted smbfs file system, and the server reboots, then the smbfs will not re-connect with the server. It will be essential a wedged file system, causing all to access it to hang.
"umount -f" does not fix this. Neither does "kill -9" of the mountd process responsible for the FS.
The magic command is:
umount -l /path/to/mount