According to OWC, Apple made changes to the boot drives in the 2011 iMacs. The drives have new firmware that has a different way of tracking the drive's temperature. Without this firmware, the iMac knows that there's a drive, but doesn't know how hot it might be. As a precaution, the iMac then cranks its internal fans to run full-bore--6000 rpm, which creates a hard-to-ignore roar. We contacted Apple for confirmation and a comment about this, but we haven't yet heard from the company.

We went through the teardown process detailed above to access the hard drive and replace the standard 1TB Seagate Barracuda drive with our own 2GB Seagate Barracude drive. When we booted the iMac with the newly installed (and non-Apple provided) Seagate drive, the fans did indeed start running loudly.

is a $10 application that installs as a preference pane to allow you to adjust the speed of the internal fan and to set temperature thresholds. We used this software to control the fan. By default, HDD Fan Control sets the speed of the fan to 1000 rpm, which is much quieter than the 6000 rpm that the fan spins at without the utility.

To get the iMac working so I could observe the fan, I used HandBrake to convert a DVD file that was ripped to the hard drive. HDD Fan Control sped up the fan to keep the temperature down.

As reported by OWC, the iMac with a drive you've installed yourself will not pass the that is bundled with new Macs. (It's on the applications disc.) Failing the test may not be a big deal, since we know it's failing, but the AHT is frequently used by Apple certified repair folks. Failure of the AHT could mean that issues usually covered under the system's warranty may no longer be covered.