Yesterday, Other World Computing wrote a post suggesting you could not install third-party hard drives in the new Sandy Bridge iMacs. According to OWC, any change in the hard drive or switching to a hard drive bay would cause the drive fan to spin at maximum speed. Though this problem does exist, it may not be as troublesome as everyone first reported.
According to the developer behind HDD Fan Control, this is a problem that has been present in iMac models since late 2009. The dev wrote his HDD Fan control application after he replaced his own hard drive and the fan speed for the drive increased to 6000 rpm. He used the hard drive's S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology) interface to control the speed of the fan and bring it down to a normal level. While Apple may not want you to replace the hard drive in your iMac, you may want to try this software option if you decide to crack open that all-in-one and do some hardware hacking. If you test it out, let us know how it works.