Can't umount when busy

[Frank Reichenbacher]

I'm trying to get mirrordir to work on a new second hard drive. I
created the partitions, set up the filesystems as needed, and then made
the directories for the mount points. I'm having a hard time testing and
troubleshooting, however, because I can't get umount to work on the
mirrored drive partitions because no matter what stuff I
close/stop/delete I can't get them 'unbusy' unless I reboot. How do I
find out what processes are tying up the mount point? Man umount says it
might even be the umount command, if so, what on earth would one do to
get around that?

Frank