Ok, so I just upgraded my 27" i5 from a seagate to a hitachi drive (7k3000 3TB) and got the thermal sensor to work believe it or not. Everyone said it was not possible, but with a little trial and error, I nailed it and my imac is working great and getting accurate HD readings.
Here is what I did:
I took the existing seagate cable out of the imac and noticed that there were only two pins inside the 4 pin cable end that goes into the back of the hard drive. The two pins mapped to the two pins on the far right with the clip facing up.
On the back on my Hitachi, there are only two pins, so I broke the cable end apart using a thumbtack and removed the two female connector sockets from the cable end. So now I have two free female pin sockets on the end of the cable that goes into the hard drive. I then took some pliers and very carefully crimped down each female socket so that they would attach snugly to the two pins on the back of the hard drive. Next I took very very carefully attached those female sockets to each of the pins on the back of the drive. My first try I put the black cable on the right side and the grey cable on the left side. Fired up my iMac and had no luck - the fans still went crazy and the temp reading was nill.
Then, I switched the female sockets to the other side, with grey on right and black on left and boom!! It works!!!!!!!!!!!!!!!!
Lastly, I put a 1mm x 1mm piece of electrical tape between the two female sockets to prevent them from touching and shorting.
To all those frustrated people out there that had already bought their drive only to find out about the thermal senor cable issue after the fact, you are in luck!!!!!!!!!
Cheers!