There are a number of reasons and conditions that can cause a disk to be unreadable. if its 3 xbox's, then its somthing to your doing to them. Attempting to mod them, leaving them in dust filled areas, putting a bad disk in, etc. I hate to say it but if its occured for 3 xbox' s for you and you still havnt managed to figure out the link between them, then theres no hope for you and you'll likely break the 4th. get a cartridge based system like the DS or 3DS.