Try running
For what it's worth, this sounds like normal behavior. Once the nfsd daemon
fires up and begins sharing that directory, it caches certain things in memory
and treats the share as being 'open'. No matter if the remote box closed the
connection.
If 'lsof' doesn't reveal any open files in the mount point, try using the
'sync' command, which writes any changes to file systems, and then umount.
NFS wasn't meant traditionally to share removable devices, so I would call
this more of an 'oversight' than a bug.
-- Shade