(Have also replied in the Galaxy Gitter - bgruening/docker-galaxy-stable - Gitter)
Having a quick read of the problem, I’m going to assume you’re moving from one Ubuntu machine to another one. My gut feeling is that there might be some underlying differences between the machines and the way the external drive is mounting.
Check that you can read/write into other folders on your external drive on the new machine. If not, it sounds like an I/O error and you’ll need to have a dig into the mounting side of things.
Otherwise, see if you can read/write into the external drive, log into the container and make sure you can run some basic bash commands. If you can do that, make sure PostGreSQL fires up and you can create new databases.
(Sorry I can’t dig into the problem any more right now)