On the new iMacs, the slots are arranged vertically the 'matching' needs to be slots 1 and 3, and/or 2 and 4. The slots used to be two rows with two slots next to each other and you matched two modules horizontally. You've certainly tried a lot of configurations my comment here would be to address the new configuration of the slots in the new iMacs. Is there a possibility that the Bank0=16GB and Bank1=4GB is the culprit? They were the original memory in the mac mini and I had no problems with them there. My diagnosis is that the 2 2GB DIMMs are bad. It's been running without problem for longer than any other iteration. I have now removed the 2 2GB DIMMs from the iMac and put the 2 8GB DIMMs such that my configuration is now: I changed the memory installation configuration to this to put match pairs of DIMMs in Banks.At least it wasn't the blue screen of death 🙂 but only because it wasn't blue. While I wondered about this, the machine crashed. This is not optimal, and I wondered if it could be causing the problem.

I discovered the system report and found that I had the following configuration: However, in short order I had an application crash-never happened before. I installed the two 8GB DIMMS in slots 1 and 3 and the 2 2GB DIMMs in slots 2 and 4. I couldn't find information identifying the memory slots vs Banks/DIMMs, so I guessed.It has been running without problems since (a couple of hours). Installed the 2 4GB DIMMs in the mac mini.Removed the 2 8GB DIMMs from the mac mini.I stored the original 2GB sticks in the Crucial packaging. I upgraded the mac mini with two 8GB DIMMs purchased from Crucial. I also have a late 2012 mac mini which came with two 2GB DIMMs. I believe that these are Bank0/DIMM0 and Bank1/DIMM0. IMac came with 8 GB of Ram, one DIMM installed in slot 1, the other in Slot 3 (using my numbering). This is basic information and I can't find it.) When you have the iMac face down with the stand toward you, number the DIMM slots 1,2,3,4 with 1 nearest the base of the machine (amazingly, I have failed to find an illustration of which physical slots correspond to which Bank/DIMM slots. I have upgraded the memory in my new 27' late 2013 iMac and it has been an interesting experience.

Imac 27 late 2013 ram upgrade