You need write permissions for the directory you attempt to backup to, in this case, your trying to write to your cd mount point.
** Changed in: rdiff-backup (Ubuntu) Status: Unconfirmed => Rejected -- [apport] rdiff-backup crashed with OSError in chmod() https://launchpad.net/bugs/86631 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs