ReadyNAS OS 6 Storage System

I am only backing up about 700GB. I have 8.2TB on this readyNAS device. The backups are daily and overwrite the previous day's backups. So, why is this device running out of disk space? I check and there's never more than 700GB of space used. At least, that's all the dashboard shows when I check the Shares. What is going on with this thing? Is there something that doesn't show up in the Dashboard? Any help would be greatly appreciated!