Keeping them or deleting them is up to you. Most people would keep at least 10% free for performance in the back up drive. I myself would keep at least 2 backups and if I had the space the first one also as a reference point. However it is up to you how many you keep but a minimum of 2 is advised. If the first one is corrupt when you come to need it then you are still in the same boat as if you had never purchased the hard drive.