Then I checked how the other sticks behaved. I repeated this for the other sticks, finding that stick 2 worked in slot 3 and four, but never 2. For me, my stick 1 worked in slot 1 and in no other slots. I started with which sticks would boot when installed in each of the slots. He suggested that I do a return and start over again, but my stubborn ass wouldn’t let it go. We agreed that the best way to pursue this was to cycle sticks to see if there was any way to get it to boot. Checked that the sticks were flush after re-installing the carrier every time.Īfter it worked (fleetingly), I assigned each stick a #, 1-4, based on which slot they were in. If I mixed in ANY combination of OEM chips, with Slot 2 always filled by a OEM stick… the machine booted just fine

Slot 2 was the most problematic, because I could only get it to boot with certain chips in Slot 2. I could get things to work with up to 3 sticks (1, 3, 4 and 2, 3, 4), but never (again) all 4. tl dr: I solved this by permuting all of the possible combinations of sticks after labelling them 1-4, based on that 1x boot order. The machine booted 1x and after that, black screen booth. I had this problem when replacing 4 OEM 16 sticks with 4 OWC 32 sticks. I bought 2-16gb Corsair Vengeance RAM as outlined above and configured them like this: I had 2 -8GB OEM RAM chips installed in Slot 1 and Slot 3

I tried several re-installs/configuration changes and confirmed all sticks were seated properly but the problem persisted until I found this thread and John's post. Only problem was, it didn't work initially and was throwing my 2017 iMac 27 3.4 i5 (18.3) into an endless restart/reboot loop. The new configuration would bring me to 48gb of total RAM. I purchased the Corsair Vengeance 32GB RAM: the author suggested via Amazon and installed 2x16gb sticks into blank slots 2 and 4 (I had 2x8gb OEM sticks in 1 and 3). , on the ease/$ savings of upgrading RAM in a 2017 iMac 27'. Oh my God! After a weekend of trial and error and lots of online research, John's configuration worked for me!

Add ram imac 27 late 2013