Hard Drive replacement compatibility

Your system has a SATA II (3.0 Gb/s) SATA HD port. So you will need to make sure the drive you get supports SATA II to work properly.

Here is the IFIXIT guide to replace it: iMac Intel 24" EMC 2134 and 2211 Hard Drive Replacement.

I think you got confused by the newer version of iMac which uses the drives internal thermal sensor to manage the systems cooling system via SMC. In your series the thermal sensor it attached externally on the drive case. You can see it in Step21.

As for replacement drive I would recommend getting a Seagate SSHD: 1 TB SSD Hybrid 3.5" Hard Drive This drive offers auto sense technology so it will work in your system. Here's the drives spec sheet: 3.5" FireCuda SSHD note the line: SATA Transfer Rates Supported (Gb/s) where we can see all three SATA speeds listed 6.0/3.0/1.5. What ever drive you put in needs to have SATA II (3.0 Gb/s) listed in the spec sheet otherwise it won't work.