I replaced my internal HDD on my 215" iMac, but the iMac can't get access to the new hard disk's (Seagate Barracuda Green) temperature sensor. The HDD fan in the iMac steps up the RPM, and over about 15min reaches 6000RPM (VERY loud) and stays there until machine is shutdown or put to sleep.

I found a fix for this: http://hddfancontrol.com/HDDFanControl/H... - this tiny piece of shareware software allows the iMac to see the temperature of the drive via S.M.A.R.T., and therefore the fans react according to the correct temperature again!