Apple - How can I keep my fan from staying at full speed after replacing the hard drive?

Macs Fan Control can set the fan of an iMac (or MacBook, or other hardware) to any speed. The coolest thing that it can also set a fan to sensor-based control depending on your new HDD by reading its S.M.A.R.T. temperature.

This free app is much better and powerful than HDD Fan Control, which is currently deprecated but still costs $35. Macs Fan Control also works under Windows in Boot Camp.

You should definitely try resetting the SMC, since it's a known fix for fan issues. Here are the instructions: Resetting the System Management Controller.

I Just downloaded the Macs Fan Control to fix a 2010 27" iMac. I have taken my silver beast apart about 5 times, reseating the LCD temp sensor connector, reseating all connectors, SMC resets, NPRAM erases, flex capacitor reboots, and praying to Steve Jobs collective soul to get these damn fans to slow down slow enough so I can actually hear these friggin speakers.

Macs Fan Control allowed the constant rate set for my 2 trouble fans, the CPU fan and the ODD fan. I now have to monitor my temps to adjust my fan speeds, but I don't crush big math all that often. The free program beats shelling out $800 for a new logic board for a bad temp sensor.