Make sure the volume isn’t in use.
sudo lsof +f -- /mnt/use_your_mount_point
Kill processes pid
kill -9 pidNumber
umount
sudo umount --verbose /mnt/use_your_mount_point
sudo lsof +f -- /mnt/use_your_mount_point
kill -9 pidNumber
sudo umount --verbose /mnt/use_your_mount_point