Hi, This is a question for a hardware specialist: I have an early mac pro: Processor: 2.66GHz Xeon (Dual-core 5150) *Upgraded to 2 X Quad Core 3.2Ghz (March - 2014) Serial #: #CK340GSUPZ Model #: A1186 Year : Aug 7, 2006 - Jan 8, 2008 OSX 10.7.5 Recently I tried to upgrade the memory from some older combinations of 1 and 2 GB DDR2 FBDIMMs - 667 MHz memory modules to: 4 x 4GB DDR2 800MHz PC2-6400 FBDIMMs to make a total of 16GB. (I don't think the makes should be relevant, but can supply if needed). They came in matched pairs of 4GB each and I placed them as recommended with the first pair in slots 1 & 2 in the top riser A, and the 2nd pair in slots 1 and 2 in riser B. When I tried to boot, the power switch started to flash and it would not boot, which (I think) signifies a memory problem. So I tried placing all the DIMMS on the same top memory riser A, even though this is not the recommended configuration - same result. I then tried just placing a single DIMM of the new 4GB in slot one of the top riser A and it booted ok. I then went back and tried all kinds of combinations, including mixing some of the older 1 and 2 GB DIMMS with the new 4 GB DIMMS, but always in matched pairs, top and bottom. I also tried putting the old memory back in as I first found it - before any boot problem. But now. NON of the memory works at all in any combination and the risers have the red problem alert LEDs all lit up. So I'm thinking that either I might have fried all the DIMMS or even worse, damaged the memory riser boards, or both. (And I'm really hoping that I didn't also damage the logic board in any way). I live in Salvador, Brazil where there is no reliable Apple tech service, so I have to try and resolve any issues myself. Any help, tips, info would be a great help as I am a musician / composer and I make my main living via my Mac Pro.