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Hi just wondering if I need a thermal sensor if I upgrade my 5400rpm HDD to an SSD?
No you don’t need any thermal sensor in your system. Here’s the guide to install it iMac Intel 21.5" Retina 4K Display (2017) Hard Drive Replacement Make sure you get the proper tools as outlined in the guide. The 27” uses a 3.5” drive which does require a sensor in the older systems. The 2014 onwards doesn’t need it with the newer macOS releases.
