I just took apart my iMac 27" late 2009 (even the cpu and applied new thermal paste).
Replaced the 1TB Hitachi SATA drive with the Western Digital 2TB Caviar Black 6Gbit SATAIII - the iMac late 2009 does not support the 6Gbit SATAIII. At first I thought it was the 3Gbit SATA cable that was in the iMac, then I swapped the SATA Cable for a 6Gbit cable still no luck. Therefore I had to put a jumper in the HDD to change it to 3Gbit. That worked well and no issues
Took out the Super drive and replaced it with an SSD (Runcore V 2.5" 240GB) and caddy to hold the SSD. - No issues with compatibility and runs at 3Gbit.
Temperature is what we all want to know.
The 1TB has the temp sensor cable directly plugged into the HDD, so if you disconnect it the HDD Fan goes crazy. The new 2TB Caviar Black does not have a temp sensor to plug into.
The Superdrive has a Thermal Temp Sensor that is stuck on the outside of the drive.
My solution was to swap the Super Drive's Thermal sensor with the 1TB's Thermal sensor. Reason being the HDD actually gets hot so I need the iMac to monitor the HDD's heat and speed up the fans when required.
The SSD doesn't get really hot, so I disconnected the Super Drive's Fan. Other than that, there's no fan issues anymore.
It's all working fine now BUT I could be wrong as maybe the SSD will get hot?
That's 1 out of 3 fan not usable as I've disconnected it. Any ideas?