Back in 2012 when the newer thin series cases where introduced Apple went two different directions from what they had been doing in the iMac’s. The 21.5” models went with the smaller laptop 2.5” HDD’s while the 27” models continued with the bigger 3.5” HDD’s.
To add to it Apple leverages the HDD thermal sensor as one of the inputs to the systems SMC services. Apple did their own thing working with the drive makers they where using getting them to customize the drives so the sensor could be accessed the way they wanted to. So you need to use the Apple OEM drives or offer your system a thermal sensor to satisfy your systems SMC with the needed input (27” models).
21.5” iMac’s 2.5” drives are a special case! As Apple altered the SMART output the drives they where using. The issue here is the SMART required an IRQ to read the thermal value (which slows the drive when it was used). As Apple wanted to access the value much more frequently than what the IEEE/EIA standard group had envisioned would be done. Apple again did their own thing forcing the HDD makers to customize the drives SMART firmware to Apples specs. Apple did propose the change to the IEEE/EIA standards body and while it took awhile the current crop of 2.5” drives (HDD & SSD) offer the needed alteration. Apple also updated the 27” iMac’s firmware to also use the newer SMART services in the 2014 systems onward after getting the newer firmware within the newer macOS releases. So they no longer need the inline thermal sensor as long as the drive offers the newer SMART services.
So the bottomline here is what is your macOS release as well as the drive you are installing?
You likely don’t need to worry about any of this!
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