I've figured it out. I looked at the core Francois Tigeot provided
(sorry if others were provided before, it was on my list!)... in
anycase, there is a bug in the HAMMER flusher which can cause it
to loose track of the flush sequence number which umountall can
trigger due to the extra flushes hammer does on unmount.
Commit e86903d84f840af38d1b452a6a6c624702373751 should fix it.
-Matt
