Hi Mayer...
Sorry for the late reply! OWC discussed this issue back in 11/09 regarding the drive temperature sensing in the late '09 iMacs. It turns out that replacing the original drive with the exact same model wasn't always a fix!
OWC pursued it a bit further and found that if the user wants to replace their iMac hard drive it is 'at their own risk'. We provided links for the original hard drive models and also links to purchase the heat sensing cable as a courtesy to our readers.
Take a look at the article and the following comment thread at:http://blog.macsales.com/2751-proprietary-cable-can-put-the-brakes-on-upgrading-late-09-imacs
Hope this helps!
OWC Angela