Apparently if you swap your 1TB Seagate for, say, a store bought 1TB Seagate with the same model number but which doesn't come with the needed Apple firmware... your iMac cannot get the correct thermal data from the jumper connection and you might have fan speed problems. One user reported that 'the fan speed increases to full over a time period of 10 minutes'. Some people have fitted old iMac external temperature sensors, some have gone for a zero ohm resistor. Again, I am quoting.
I found a further post, though it it's not clear which model of iMac the user is referring to beyond that it is an 'aluminum' model, where the user bought an optical drive sensor (Apple part #922-9141) and used it in place of the original HD sensor. He says it plugs into the logic board where the old HD sensor did and that you stick the sensor onto the hard drive with adhesive that is already on said sensor. That returned the system fan's behavior back to normal.
