Hello, I have a quad-core 2.66 Mac Pro 2009 Nehalem running Lion. For over a year it has been running with 24gb with no issues: 2 x 4gb Crucial 2 x 8gb OWC After a while I started to get startup issues and other weird behaviors, and after several switching and reseating it turned out one of the OWC modules was causing the error. The Mac kept running fine with 16gb (2x4 + 8) while I ordered two new 8gb modules. I was hoping to use a 3x8gb config, and maybe add one of the 4gb modules in the last slot for the unusual total of 28gb. However, the Mac refuses to boot (flashing light) when I mix the new modules (Samsung) with the OWC module. So, with the Crucial modules out of the equation: 1 x 8gb OWC -> Startup OK 2 x 8gb Samsung -> Startup OK 3 x 8gb (all of the above together) -> Flashing light I've tried many different combinations, module order etc, and even tried to mix the Samsungs with the Crucials, nothing helped. I've also tried the system reset procedure recommended by Apple (keeping the power cord disconnected for 15 seconds etc). Any ideas what I could do next? I've just invested in two 8gb modules only to find myself at the exact same RAM installed as I had in the first place... Thanks.