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I recently purchased a new RAM module for my new 2020 iMac. I went with this model from Crucial since it seemed to fit the specifications from Apple's website: https://www.crucial.com/memory/eol_ddr4/ct16g4sfs8266

However, when I received it and put it in my iMac, it fails to boot. I've tried it in every slot in multiple configurations, when this module is in the computer, it won't boot. I'm aware of the memory initialisation process, so have been waiting longer than 30 seconds before powering off.

After doing some Googling, I came across this model from Crucial on Amazon that many people report working on their iMac: https://www.crucial.com/memory/ddr4/ct16g4sfd8266. For clarity, mine is the ct16g4sfs8266 and this is the ct16g4sfd8266. One character difference!

Other than the model I purchased being EOL, the specs for these two modules on Crucial's website seem to be identical. They are both DDR4-2666 SODIMM memory, so I'm failing to see what might be causing the module I bought to prevent boot. Does anyone have any experience with either of these modules? Did I just get unlucky and get a bad module?

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  • Why didn't you just use Crucial's configurator? iMac 2020 RAM listed here - crucial.com/compatible-upgrade-for/apple/… Your code isn't on that list. The chip you got is EOL, I checked its replacement for compatibility. … Not compatible. The alternative part in your link is also not compatible. – Tetsujin Sep 19 '20 at 6:43
  • Thanks for that link! I guess what I'm trying to understand is why it's not compatible. From what I can understand of the requirements Apple publish, it should be. – rheotron Sep 19 '20 at 7:32
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    Honestly, I don't know - but that's why I never choose my own RAM by the 'spec' itself ;) Last time I did, I got it wrong, same as you… though by heck they looked alike. The company I bought it from very graciously swapped it for the right stuff, without actually calling me an idiot [& paid the postage both ways across the Atlantic too] ;-))) – Tetsujin Sep 19 '20 at 9:18
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    Following up for anyone else that finds this in the future. I went out on a limb and bought crucial.com/memory/ddr4/ct16g4sfd8266. Works like a charm. The chip layout looks completely different, but other than that still don't know what the issue with the previous module was. Thanks for the help! – rheotron Sep 21 '20 at 9:47

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