Lets see if we can get the facts straight here:
Apple has had a temperature sensor on their Hard Drives for quite some time. Over the course of the years Apple has migrated this sensor from a stuck on unit on the drive, to one that is based within the drive its self.
The Mid '11 models are the first models that used the internal temperature sensor. Apple contracted the HD manufactures to use special firmware on their drives so this sensor was accessed by the systems heat management services which controls the speed and running of many different fans within the system.
So what happened here is many people encountered issues when the fans would ramp up to full speed as the replacement hard drive people had used did not have the needed sensor and/or firmware to communicate to the heat management services so the system went into fail mode putting all of the fans into high. Of course this was not good as the fans were not designed to stay at this high RPM for long nor was it pleasant to hear.
Some people tried a software hack to trick the heat management system leveraging the SMART system heat sensor on the newer drives. But this came with a cost as this required a hardware IRQ request stealing a bit of system performance depending on how often it was polled. In addition it was slow in reacting if the polling was not to often. If you didn't have a drive that had a SMART thermal sensor you could manually set the fans to a set speed. But that too was not to good as you are now using a fixed amount of cooling so the drive or system could still overheat.
A better solution was via a hardware hack basically putting back the external sensor on the Hard Drive and splicing into the drive cable the sensor. This was the better solution for quite a while. OWC still offers this part (we bought a good 50 of them!)
After awhile the Hard Drive manufactures were able to put out drives to the marketplace that had the needed sensor and firmware to work with the newer iMac systems (with Apples blessing). So you can now get drives that don't require any hacks!
You will need to do your research here as there still is a lot of older drives out there that won't work. If you get a boxed drive look for the Apple logo on it you will be getting a drive that will work. In any case I strongly recommend buying from someone you can get your money back if you get the wrong one.