Why is my drive displaying unable to do so when my HD drive is half the space I'm backing of the easystore?

I have a 1 TB portable Easystore, running Win 7. All of my utilities check out with the drive pass ok. It appears that the backup is working but I’m not fully sure because most of them seem unfamiliar and I may have forgotten about some and a lot to check including system files etc. I am also not really sure if I am doing a valid check. I have done a “mock” restore and they seem to be displayed there…BUT when I enter the backup screen, at the top, there is an indication statement…“Unable to back up files to Easystore because there is not enough space.” Funny because my total HD space I am trying to back is only about. 4 gig of used space and less than 7 or 8 gig most at total. It also displays that my last backup date/time was not indicative of the actual one I had performed. And these indications never change when I perform a new backup. Anyone have a clue? Would appreciate your response and advice. I had upgraded to the latest WD had available and they are no longer supporting.it.