Original post by: Nick Butler ,
I purchased https://www.applecomponents.com/items/0000004398/cable-temp-sensor-optical/?pn=1 which is a Optical Drive Temp Sensor, and it works.. tape it to the HDD just to the upper right of the sata plug on the back of the drive.. on the black aluminum; middle of the drive.(according to seagate, thats the optimal place for a temp sensor) and voila.. the answer.. How I came to this conclusion was, I originally took the optical drive thermal sensor from my ODD and put it on the HDD thinking this should work.. well the HDD temp and fans work perfectly.. and the ODD fan is at full blast.. so I tried to contact APPLE for the replacement ODD temp sensor.. Mistake.. and then I found the Link above.. and used it as a replacement.. TADA!!!