I have an iMac 27″ (2017), which shipped with 8GB memory (2x DDR 4 2400 8GB). I upgraded it to 32 GB (4x DDR4 2400 8GB modules, Patriot Viper) when I got the Mac, and used it for 2 years without any problems.
2 years ago, I saw a deal of Crucial 32GB (2x DDR4 2666 16GB modules). I thought that I can install them into this iMac and then repurpose the Patriot Viper memory modules for my Intel NUC mini-PC. I remembered removing the 4 modules form the iMac is pretty hard, there’s no way to grab them firmly. But other than that I remember the replacement is success and the iMac is stable for a while (greater than 6 months).
And then for almost a year the iMac starts to develop stability issues. It will restart abruptly (showing different kind of kernal panic), showing four languages what not. I was thinking maybe the fans are fault or maybe the mainboard, graphic cards developed age issue. I also suspect that the Mac OS have too much garbage in the libraries folder.
And then couple of months ago the Montery upgrade breaks the APFS partition, leaves a huge “black whole” of disk space. There was 512GB SSD but visible capacity is about 200GB. It turns out that’s some weird APFS hidden volume bug during OS upgrade, which may related to iPhoto’s photo caches. That’s OK, I can do time machine and partition from scratch to address that issue. But that addressed the disk issue, but the system is still restarting every other day.
One day, I thought I should try to replace the memory modules, because that might be in fault with all different kind of kernal panics. Maybe the older iMac is not suppose to run the memory at 2666Mhz, that might be a issue.
Then I pulled the 4x 2400 modules, pushed them to the iMac, restart, no boot. Then I pull 2x of them out, leave 1 & 3 modules in. And then the iMac boot flawlessly (note, you need to wait about 20 seconds before the iMac can boot when you press the power button). Then I run couple of stress tests (stressng), it’s pretty stable. I run the system for couple of days and it stays stable! So maybe my hypnosis is right and I should use 2400 modules.
Then I tried to push the 32GB 2666 modules in, mixed with 16GB of 2400 modules. That won’t boot. Try 2 & 4 modules with the 16GB modules, won’t boot. Try 4 modules of 8GB modules again, won’t boot. And then to make sure I captured the ghost I installed the 32GB 2666 modules at 1 & 3 again, oops, it booted. And then for couple of days, it’s stable.
OK, I think the problem is resolved. But I have no idea why! I seems a year of stability issue might be some electric conduction issue? Anyway, it’s hard to “reproduce” this kind of issue sometimes. So the IT solution of “kick it and restart” might works.