The imac hard drive has an integrated temperature sensor the imac fans will run at full speed if you replace the hdd without the temperature sensor and or proper connector.
Imac 2015 ssd thermal sensor.
Install the ssd into the hd bay and download the ssd fan control application to avoid fan noise easier but less legitimate install the ssd into the hd bay with the owc thermal sensor extremely difficult considerably more expensive but works properly install the ssd into the optical drive bay loose optical drive but no fan issues.
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I have fitted a 1tb wd blue ssd to my late 2015 imac 27 it had no hard drive before just the pcie ssd.
This hack fix bypasses the hdd temperature sensor in order to fool imac into thinking that hard drive is working at normal temperature.
Imac hard disk removal imac ssd upgrade thermal sensor link posted under description imac 27 harjindersidhu sidhudiy easysteps use screwdriver t8x50.
All i needed to source was the sata power cable.
I did not fit any thermal sensor and it works perfectly no fan spin or any other problems.
This will be required for any 3 5 or 2 5 drive.
I also replaced the pcie ssd with a samsung nvme 1tb unit with an adapter this also works perfectly.
I think that owc thermal cable for imac 2013 and later is a scam.
I would use this as a workaround and stil.
Also this version of the 27 imac imac14 2 doesn t have the emmc slot on the logic board so i couldn t get say a 64gb emmc drive to do the fusion drive upgrade.