Improvement: faster check for existing copies on --going
When confirming that the files on the drive that is going also have a copy somewhere else, we don't need to know how many copies of the files exists; as long as one exists, we should be fine. So when we find a copy, we stop looking.
Bugfix: don't log errors about not being able to know how much available space a unmounted drive has.
It won't be used anyway.