Backup of16.04 gave 240gig back, then ran out of space

I did a Backup of16.04, it ran out of spac 240gig. There isn't much more to say. I created a folder in my 1 terabyte backup storage device. I set the path to the folder in Déjà Dup set up. I checked out the folders to include and those to reject then set the backup to run. It took ages to complete and told me that it had run out of space. I checked the location on the 1 terabyte backup storage device and the location was full. So I know the target was correct and backup data was sent to ithe location. At the start there was 342.3GB showing free on the backup device. I have tried this three times using other locations and the same problems occur each time.