I have a 1TB My Book external drive. I have had it for about ten months. I thought it was merrily backing up my computer. The other day I checked it and I had a whopping 350MB of free space. I did a disk clean up and now have 54.9GB free space. I have 80GB shown as total data in my source computer. In the Additional Files area of the Backup drive it shows 857.8GB stored there. Can someone please tell me what this data is and how do I ,or do I want to, get rid of it? I have looked the manual over and can’t find a way of identifying what it is that is stored there.