NVMe M.2 drives do work with the iMac 5K 2015 and 2017 as well as the Macbook Air 13" 2015 and the Macbook Pro Retina 13" 2015 out of the box with the correct adaptor. It appears that only particular versions and brand of the adaptor works well.
The 2013-2014 Macs need a modded firmware to work well.
@danj has mentioned in another thread that adaptors might have massive CRC issues, but he did the test 1 year back. No sure if the adaptors that the Macrumors users are using are newer versions that seem to work very well.
Check out this link. Someone used a Samsung EVO 970 drive in his iMac 5K 2017 without any issues.
The HDD is upgradable without a thermal sensor if you use a temperature control app.
You can use Macs Fan control or SSD fan control. Both will work.
The SSD only version comes with a 3.5" Hard Drive bracket but not the SATA cable.
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