Insufficient Back-up Disk Space

Hi Guys I save my backup every week to a dedicated 1tb internal back-up disc (one of four HDDs). All of a sudden my weekly back-ups are not completing because of insufficient disc space (179gb available from 931GB). When I go into manage space to clear backups I have only 2 backups to chose from; both around 170gb. I looked at the HDD and I cannot work out why so much space has been...