Also a screen session can cause this - easy to forget they might still be hanging around
IF you are sure nothing important has a share mounted (which could be anything, stuck session that is no longer, etc). I just ssh to my unRAID server and execute:
ps -ef | grep /mnt/user
I then take the PID and perform a graceful kill (I've only had to do kill -9 once)
kill <PID>
as soon as all mounts in the process go away, your array stopping will be successful.