2k3 Tape Backup

I have setup a daily full tape backup that we rotate between two tapes. I’m using Windows Backup Utility and when it preforms the backup I get the following error: “Cannot locate the specified media or backup device. This backup operation will terminate”. If I mark the tape as free then the backup runs. I need the backup to override the information on the tape automatically because I’m not at the office every day and I may not be able to remote in.