We have a 2009 Quadcore 2.66GHz Mac Pro that absolutely refuses to update past 10.6.2, producing kernal panics immediately after the update when trying to boot the newly installed OS, be it 10.6.3 or 10.6.4. The Mac Pro has 2 NVIDEA 120s/driving three 19" monitors - 8GB (4x2GB) RAM, and we are using a third party keyboard and a Kensington track ball. What do you think could be the cause, it happens on both a cloned 10.6.2 system updated and a clean 10.6 system, immediately after the update. Could a bad RAM module or Video card cause a kernal panic like this? Both memory and the video cards work fine with 10.6.2. The Mac Pro also has 2 DVD drives, one is stock, the other is a Pioneer Blue-Ray/DVD combo drive. There are 4 HDs installed, and the kernal panics literally destroy everything on the updated disk (tried several different HDs, all the same result), requiring a clean format with Disk Utility to get the HD usable again. Arggg... driving me crazy. Any advice much appreciated.