I have a MyBook external drive which is almost full 2.6 terabytes are additional files. My PC hard drive is only 500g. I am checking each file through explorer and so far have not found a folder that large. If I do find the correct folder is it safe to delete the contents. From reading other posts I am guessing it must be some kind of multiple back ups.
I would suggest you to check the computer and data for any Virus or malware.