How do I force an autofs mount to unmount if it’s currently busy (and non-responsive)?
I’d try stop Kodi first (sudo systemctl stop mediacenter) as it’s likely the service occupying it.
If stopping Kodi does not help, you can try:
sudo umount -f /mnt/<drive>
But if it’s really busy then that will most likely fail.
You can also often find out what is being done on a mount by using lsof. You’ll need to install it first with:
sudo apt-get install lsof
Then try something like (replacing the path with the actual mount path):
sudo lsof | grep "/mnt/mountpoint"