Comment 121 for bug 672177

The best way to try and figure out what is causing this is to modify /etc/init.d/umountroot and right before the umount's, add

/usr/bin/lsof -n > /saved.root.lsof
sync

This will save a listing of all opened files and which processes have them open. Things marked as 'deleted' in this list are generally the problem. If you see libc6.so opened by upstart, then this bug has regressed. Otherwise, it is probably something else.