There is a simple solution to this, we have done it hundreds of times for the 2010 iMacs. You just replace the hard drive temperature sensor cable with the optical temperature sensor cable from any 2011 iMac, it sticks on to the new hard drive and works perfectly, even passes Apple's own Mac Recource Inspector diagnostics.
Bit of a long shot, but figure it can't hurt to ask.
Have a Late 09 iMac at work which has, among other problems, a bum HDD. A new 1TB drive from Apple is like $350, so what I'm looking to find out is whether or not the thermal sensor that is built into the drive on these models, is in any way specific to the drives Apple supplies.
I'm inclined to think that I could use any drive so long as it has the same 4-pin sensor spot on the drive, because it would be rather expensive to get HDD companies to produce a bunch of drives specifically for Apple when the only real difference is that thermal sensor. As you might predict, Apple reps refuse to be pinned down saying yes or no, just basically saying that they only recommend using Apple parts.
The system is out of the Apple warranty period, so there's no issues there. But between the HDD and logic board it needs due to a bad SMC chip, it's basically going to cost the same as giving the person a new system.
So anyone out there ever try putting in an aftermarket drive into a unibody iMac, and have any knowledge as to whether or not A) there are thermal sensor pins on aftermarket drives, and B) if they would be readable by the Apple firmware? And before someone suggests it, I know I could simply bridge the connection points for the sensor readout and that would likely keep the fans from running full speed, but it's not a very professional solution. It also has potential long term side effects of not having the fans kick up the speed when it's actually needed.