27" iMac Hard Drive Upgrade 2TB WD 2001FASS Temp Sensor ?

I can't give you all the details, but I can say that around the time of the 2009 models, Apple switched from using a sensor that was basically just glued or taped onto the drive to one being built into the drive. And while you could always choose not to connect this particular cable, it will cause your HDD fan to default into running at full speed all the time.

I have not seen any other drives, except those sent to me by Apple (I'm an ACMT) with those extra pins for the temp sensor, and a cursory look over the drive looks like there's a little extra PCB compared to other drives.

I can't guarantee it would work, but if you could get your hands on one of the old sensors from systems pre-2009, you could always see if that would work as a substitute. I'd say it's at least worth a try, and you can probably find that cable on ebay or something.

And have you ever taken apart an iMac before? I won't say it's hard, but it does require some not so common tools to do it properly, and there are some potential safety hazards to be aware of, like two exposed power supplies once you get the display off. And that glass on the front is not tempered, so it will break into very sharp little shards if you should drop it. You'll also need a T-10 Torx screwdriver to get the display panel off. Make sure it's magnetized, and even then the magnets holding the glass panel on will have a habit of grabbing the screws and your screwdriver.

Also keep in mind that by doing this, you will be voiding any warranty you may still have on that system. Meaning if you break it, you're paying to fix it, and that will NOT be cheap. So you may want to rethink your decision not to take it to either an Apple Store or AASP in your area.