Hello, I have a MacBook Pro (15" 2.16 Ghz Core Duo) running OSX 10.5.4. I purchased the machine with the standard 1GB ram module. Recently I decided to upgrade the RAM (to a max of 3GB) and according to Apple, I should be able to install a 2 GB module in one slot, with the original 1GB module in the other. I purchased the following RAM module - Kingston KTA-MB667/2G, which apparently is Mac compliant. I've tried these tests multiple times, and I've ensured to properly seat the RAM. So here are the results: With a combination of the two Ram modules (switching upper/lower slots for each stick): No boot - I hear the disk spin up, the sleep light turns on steady, but the screen remains blank. With the original 1GB Ram in either slot: normal boot (1GB recognized). With the NEW 2GB Ram in either slot: normal boot (2GB recognized). I know both the slots are good, both RAM modules seem to function as well, so there seems to be an incompatibility when combining the RAM modules. Therefore I purchase a SECOND 2GB Ram module (KTA-MB667/2G), exact same as my new one, for a total of 4GB. The machine will only recognize/use 3GB (according to the note in the link to the Apple site above). So here are the results: I first try combining the two 2GB modules: With a combination of the two 2GB Ram modules (switching slots as well): No boot - as above. With the 1st 2GB Ram in either slot: normal boot (2GB recognized). With the 2nd 2GB Ram in either slot: normal boot (2GB recognized). Then I try combining my original Ram with the new (2nd) 2GB stick. With a combination of the original 1GB Ram module + the 2nd 2GB module (switching slots as well): No boot - as above. With the original 1GB Ram in either slot: normal boot (1GB recognized). With the 2nd 2GB Ram in either slot: normal boot (2GB recognized). I've tried every permutation - always with the same result. I'm dead certain I'm correctly seating the RAM, as the machine always boots normally when there is only one slot used. So what do I do? Is this a problem with the machine? It doesn't seem to be the Ram.... I'm completely at a loss, any feedback would be much much much appreciated.