Backup disk getting full..now what

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I am using Ghost 12 to backup a couple of data files - Photographs and images - I have this set to do a mirror backup to an external drive.

 

The 1 TB external is getting full - How do I add more drive space without duplicating whats on the first drive.  I can see myself getting larger and larger drives with duplicted data on each

I don’t quite understand your question: have you tried purchasing another drive? Or removing older backup sets? I apologize for my misunderstanding; any additional details you can provide would be appreciated. Thanks!