Comment 2 for bug 1352012

This error message indicates that you have a non-default filesystem on /mnt that's in a problematic state. The base-files package provides the /mnt directory on the filesystem, so dpkg assumes it should be able to unpack that directory if necessary. I think you will need to unmount /mnt on your system to upgrade base-files.

Marking this bug as invalid, because I don't believe there's anything we can do in base-files to mitigate this.