I have set up a backup for my 500 gb shared storage 2 using an external 500 gb usb drive. But there is a settings were you choose how many versions the backup should save before it deletes the old ones and the lowest value is 2 month! What if my backup is 450 gb, the drive will be full after the first run. Why can i not choose a lower value? What will happen when the usb drive is full?