There may be other ways to unmount a volume from a working docker instance, but using the way below you can unmount any volume right away.
On the docker instance itself, try these commands:
You will see a list of all the mounts.
overlay 310G 160G 151G 52% /
tmpfs 64M 0 64M 0% /dev
tmpfs 7.7G 0 7.7G 0% /sys/fs/cgroup
shm 64M 0 64M 0% /dev/shm
/dev/sda1 310G 160G 151G 52% /etc/hosts
/mnt/hdd2 310G 160G 151G 52% /mnt/hdd2
Now check the last column of the result and copy the path of the mount you want to remove and run this command:
umount -l /mnt/hdd2
Please note that you will have to do it every time your docker instance restarts, so please add this command to your startup scripts.