Norton Ghost 15 unable to create recovery point set

I am running Ghost 15.0.1.36526 on a pc with Windows 7. I have 2 partions on my internal drive and various external drives. I am having difficulty backing up an external 1TB drive (with 100GB data) to an external 500GB drive. I only get the option to create an independant recovery point not a recovery point set. Setting backups for the other partions works fine. I have deleted all the backp jobs, rebooted and started again but still get the same problem.

I'd be grateful of any suggestions for a fix.

Thanks