zfs not exporting or unmounting

[David Ehrmann]

I tried to export/unmount my zpool:

# zpool export tank
cannot unmount '/tank': Device busy

# zfs unmount /tank
cannot unmount '/tank': Device busy

but it wouldn't, so, after closing everything that could be using it, I 
tried again.  No luck.  Then I tried to see if I forgot something:

# fstat -m /tank
USER     CMD          PID   FD MOUNT      INUM MODE         SZ|DV R/W NAME

# lsof | grep tank
<nothing>

I'm out of ideas.  zpool export -f tank worked last time, but at the 
expense of taking down one drive in the mirror (it shouldn't have, but 
it did; I had to resilver it).

The one thing: I just did freebsd-update upgrade -r 7.1-RELEASE, and 
possibly freebsd-update install.

Thanks in advance.