Yeah, as above nothing returned by lsof, assuming fuser /mnt is the correct way to run that returns nothing either.
I have nothing open I am aware of, and wouldn't that report as something being open via lsof or fuser?
Also a screen session can cause this - easy to forget they might still be hanging around
Yup, have done this many times before so first thing I do is exit any screen sessions before trying to shutdown now