Chosen Solution
I think this guy: (Why does iMac boot up with just one memory in any slot?) has the same problem that I have. I have a Mid 2011 2.7 i5 iMac that only boots with a single DIMM of memory. I have tried many combinations, the closest I could get it to boot was with 2 identical 8GB memory sticks in the two right hand memory slots. But otherwise, it just falls into an eternal boot loop. Logic Board? EFI? Board Firmware? If it’s unfixable, this is okay, I will have to settle for 8GB of ram for now (populated in the top, right hand memory slot). the NEXT question is, I want to upgrade this Mac to the i7-2600 processor. If I can only use 8GB of ram, does that even make sense? I use photoshop and Lightroom to edit 16-30 megapixel still images (Canon/Fuji cameras), and 8GB is like… Okay at the moment, it gets by. OR should I just buy a functioning logic board with an i7 already in it, and possibly gain my functioning ram slots? EDIT 4/8/20: attached is a picture of the ram that I am using. I have two of the bottom one (timetec) , and one of them is still in the computer. I tried many combinations of the 2 8gbs and the one 2gb that I have, the previous owner was up front with the problem and told me that he had tried various 4gb/8Gb dimms that he was using functionally in his other 2011.
Other than a size difference between the two sets I don’t see anything with the SO-DIMM’s to explain your problem. You appear to have a bad logic board. The PCH and the support logic is located on the backside (visible metal heat sink)
I would take the logic board out to inspect the back side to see if there is something visible that explains why the SO-DIMM’s are not recognized. You talk about upgrading the CPU to a i7-2600, maybe its easier just to replace a complete logic board Vs trying to fix this one. Here’s one source 27” iMac Logic Board, 3.4GHz, Quad-Core i7, Apple P/N 661-5950
