Decided to try this on my 2017 21.5" iMac as I disassembled it to still upgrade your RAM. Taking apart an iMac was very tedious, but I followed the instructions I found online and took my time. The adapter and drive fit my motherboard perfectly. I was able to install macOS Mojave using the USB installer. I used a 2TB Samsung 970 Evo m.2 SSD. I'm now getting consistent read and write speeds of around 2500MB/s. That being said, this will NOT work on a base 21.5" iMac as that version does not have an SSD connector soldered to the motherboard. If you have a Fusion Drive or your iMac is specifically configured with an SSD, this adapter should work. Just take the time to disassemble your iMac as there are a lot of flimsy cables.
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