"replace the hard drive, how do I connect Hard Drive thermal sensor?" için yanıtHey, I realize this is an old post.However, I have learned some things while trying to replace the hard drive on a 2011 IMac 21.5". First, the hard drive used must support SATA connections of 3Gb/s, as this is what the model I am using uses. Anything else will result in a white screen with a flashing question mark folder. Secondary, I overcame the thermal monitoring problem by using the equipment already provided. The hard drive in the mac had failed but the board still had a functional thermistor on it, allowing me to unscrew the PCB from the old hard drive and keep the 4 pin cable attached. Then i tucked this PCB under the new drive and only plugged in the SATA connections into the new drive.