Force unmount stuck LUKS encrypted disk on Linux Mint

If Linux Mint refuses to mount an encrypted external drive that was abruptly disconnected and gives you a message like "file already exists" or something like that, instead of rebooting the system you can try unmounting it form the command line:

sudo udisksctl unmount -b /dev/mapper/luks-xxx-xxx-xxx-xxx-xxx

or

sudo dmsetup -f remove /dev/mapper/luks-xxx-xxx-xxx-xxx-xxx

Just type until luks and then press tab twice to see what's the id in your case.