According to the report of OWC (Other World Computing), Apple has added a new hurdle for users by implementing a new sensor in latest models of iMac, which detects the Hard drive temprature and prevent users to replacing with non-Apple hard drives.
“For the main 3.5? SATA hard drive bay in the new 2011 machines, Apple has altered the SATA power connector itself from a standard 4-pin power configuration to a 7-pin configuration. Hard drive temperature control is regulated by a combination of this cable and Apple proprietary firmware on the hard drive itself. From our testing, we’ve found that removing this drive from the system, or even from that bay itself, causes the machine’s hard drive fans to spin at maximum speed and replacing the drive with any non-Apple original drive will result in the iMac failing the Apple Hardware Test (AHT).”
If you have not custom 7-pin hard drive cable and proper firmware included on the original (stock) hard drives on new machine, thant the fans of new iMac’s spin to full speed and the iMac fail to pass the Apple Hardware Test.