I was trying to unmount the /opt filesystem today, but I was prompted with a "device busy" message. Frankly speaking, I thought the file system was already in an inactive state, so I was curious as to what processes were hanging it. These processes were identified and subsequently killed.
# cd /
# umount /opt (device busy)
After realizing that processes were still accessing the /opt filesystem, here's what was done to umount it.
Determine which processes were accessing /opt
# cd /
# fuser -cu /opt
# kill -9 PID1 PID2
# umount /opt
Alternately, this could have been done, too.
# cd /
# fuser -ck /opt
# umount /opt
or simply force it
# cd /
# umount -f /opt