Windows XP Backup

Im having an HP Surestore DAT 24 external tape drive and using Windows XP Pro Backup utility to back up a disk with 20GB of data. Backup works fine but after it reaches the 12 or 13 GB mark, a message will pop out that the “media is full, please insert a new tape”. Im using 24 GB DDS3 tape and compression is set to 2.22 to 1 as confirmed by the HP Storage Works utility. Please help.