Last month, I upgraded to Leopard from Tiger, and am now having a problem reading certain CD-ROMs. Each month, I receive a CD from a guitar instruction site. When I was using Tiger, I had no problems reading any of these CDs, but, since the upgrade, I've become unable to open the most recent few. They aren't even recognized by the computer, and are ejected automatically, although they work perfectly on two other Macs running the same version of Leopard. Older CDs from the same company still play fine. My drive is a Matshita DVD-R UJ-857D. Any idea what might cause a problem like this? Thank for your help.