1 Hour Demo or Full Version with License
End User License Agreement
You will find HDD Fan Control in your downloads folder.
It's both the demo and the full version (with license):
You can try it out for 1 hour after installation
then you must purchase a license to unlock the full version.
Instructions and Help for HDD Fan Control can be found in Support.
Reduce Fan Noise and Protect your Drive.
A post on Cult Of Mac reporting on the hard drive replacement fan issue in the latest iMacs shouts out HDD Fan Control as a solution, read the article here
2011 iMacs Make It Impossible For You To Upgrade Your Own Main Hard Drive