DVD - no boot

Hello everyone: I'm running SuSE 9.3 / 64-bit mode and I tried to make a back-up copy of the dual-layer DVD. According the K3D, I did just that as the file sizes were identical. However, the copies I made will not boot. If the machine is up and running and I want to install software through YAST, then the copy DVD works OK. But, if I try to boot from the copy DVD, nothing happens. What have I done/not done and, more importantly, is there a way to correct this. Any help greatly appreciated. Cheers, Mike