Reported by David Thompson <dthompson2@worcester.edu> at <https://lists.gnu.org/archive/html/guix-devel/2015-10/msg00284.html>. * gnu/build/file-systems.scm (mount-file-system): When SOURCE matches 'regular-file?', do not create MOUNT-POINT if it already exists. This fixes a bug whereby we would be truncating MOUNT-POINT if it already existed.