Are you going to replace the HD or SuperDrive? Or adding it to the "SSD slot" (only in 2010 iMacs)? If you have 2010 iMac, then I recommend you to put it in the space that is meant for the SSD. There is no thermal sensor to worry about in that method. If you have 2009 iMac, then I would recommend replacing the SuperDrive because its thermal sensor is different and can be used with an SSD (basically, you just stick the sensor somewhere on the SSD).