I got delivered today Snow Leopard and when I put the DVD into my iMac7,1 (Mid 2007) it started making a noise like something was in the lens thing and then it just ejected itself. I then tried other DVDs and it worked fine. But this Snow Leopard disc would not be read by the iMac. I then put it in my other macs and the disc worked fine. So I burned it onto a new blank disc and put that into the Mid 2007 iMac and it had the same problem. What am I ment to do? I am stuck.