How do you clean up old database and transaction log dumps without removing the backup device? I cannot find a way to delete a single instance of either.
Instead of deleting them, try the approach of overwriting the device by doing a new backup on the device using the WITH INIT / FORMAT clauses of the BACKUP command. FORMAT is required for the first use of the device, but I don't think that's your issue because you've already got files on it. I'm not certain whether WITH INIT alone is enough to wipe out the file, though, so use WITH INIT and SKIP first to see if that does it.