You are correct the 21.5” models don’t need the in-line sensor for the 2.5” SATA drive bay. But! You do need a drive which supports the latest version of S.M.A.R.T. services which is how this system gains access to the drives thermal sensor.
You do have a second issue! I’m assuming you are planing on taking the SSD drive from your 27” iMac to put in your 21.5” system. As I stated you need a newer version of there 2.5” drive (a SSD here) if you are planing on swapping it out. You may want to rethink that!
As you can’t use the 2.5” SSD with the PCIe/NVMe blade SSD your system has (A fusion drive is in fact two drives which are logically seen as one). Your systems two ports are in use! So you’ll end up swapping out your 2.5” SATA HDD which you could do. But your system performance won’t be that dramatically better! You also will end up needing to split the Fusion Drive before you do anything How to split up a Fusion Drive
So what would I do? There are two different directions:
One is to just go with an external Thunderbolt RAID drive system to hold my 2.5” SSD with a mate to create a mirror set. This would be the better direction if you have a 512 GB or larger SSD. Here you can then add in additional SSD’s (if you get a four bay box) to push things farther.
The other direction is to replace the PCIe/NVMe blade SSD to a much larger drive. Apple only puts in a small 32 GB drive in (way to small to hold a full OS & Apps and still have free space). While a lot more work it will offer much more performance than just adding the 2.5” SATA SSD alone. I would go with a 256 or 512 blade. If you really can swing it jump to a 1 or 2 TB blade! Then your system will scream!
RAID boxes:
Apple blade SSD’s:
Here’s the IFIXIT guide you’ll need to follow: iMac Intel 21.5" Retina 4K Display (2017) Hard Drive Replacement and heres the guide for replacing the SSD: iMac Intel 21.5" Retina 4K Display Blade SSD Replacement. You’ll also want this kit: [关联产品丢失或禁用: IF173-005] and you may want to order a second set of adhesive strips (just in case). Take your time and follow the IFIXIT guide techniques as its very easy to damage the display taking it off.