RD1000 Media Runs Out of Space Available, BE12 Not Cooperative in Changing to Next Cartridge

The Dell RD1000 cassettes I have can only contain 320GB capacity. The server I'm backing up has well over that amount to backup. When Backup Exec 12 registers that the backup has reached full capacity, it stops the job and prompts me to "Please insert overwritable media into the Backup-to-Disk device."

I can't replace the media, because the RD1000 drive won't release it when the eject button is pressed.

I can't tell Windows Explorer to eject the cassette (it responds "'RD1000-320GB (F:)' is currently in use. Save any open files on this disk, and then close the files or programs using the files before trying again. If you choose to continue, the files will be closed, which may cause data to be lost."

I can't run an ejection job with BE12 because it is currently running a backup, and I would have to cancel the backup to run the ejection job.

How do you back up data greater than 320GB on RD1000 media using Backup Exec?