The new 2011 iMacs make it very difficult when it comes to hard drive replacement. That may surprise a good many readers; after all the iMacs haven't had user-serviceable hard drives for many years. While Apple's policy has been hands-off when it comes to the inside of the iMac, many DIY enthusiasts have been swapping out hard drives in their iMacs for years, but starting in late 2009, Apple made it a bit more difficult by using internal hard drive temperature sensors. You could still swap out drives, but you had to use the same brand to ensure that the temperature-sensing connections were the same.
Now Apple has gone a step further: The 2011 iMacs use a combination of custom-embedded firmware within the Apple-branded hard drive and a SATA power connector cable that uses additional pins to monitor internal temperature. The SATA power connection doesn't look any different; it's still a 15-pin connector. But the old power connectors only used four wires: ground, +3.3v, 5v, and 12v. The new Apple cable uses seven wires; four for power and an additional three for temperature monitoring.
The result is that you can't replace the internal Apple-supplied hard drive without the iMac either failing its power-on test, or causing the fans to run at full speed.
Do you think that Apple has gone too far? This appears to be an attempt to prevent third-party upgrades of the iMac.