A weird thing happened after I installed the latest 10.5.6 update downloaded from apple.com. The update I downloaded is the combo 668.7MB dmg. After installation, I restarted as promted. The MP restarted, then System Profiler showed only 12GB out of 16GB of RAM installed (I have 8x 2GB, but Riser B's DIMM 1 & 2 showed "empty""). Next the sys froze when attempting to open Activity Monitor. I restarted OSX, now the screen of death at Apple logo I turned MP off, took each RAM Dimm out and physically reinstalled. Started the MPagain, same thing. Repeated RAM physical reinstall as before. Restarted and everything works --16GB of RAM. FYI, my RAM is from OWC. Any idea what went wrong? Does it sound like a software/RAM/MP issue to you?