Answer to: 24 in iMac fan running constantly after replacing hard driveHi, its a temp sensor apple puts in their hard drives thats not present in other drives not from apple. There is software designed to deal with this problem, its not free but works well. The website also explains the problem, The software is iMac HDD Fan Control, the website is www.hddfancontrol.com. Oh and there is a free demo to see if it works for you. EDIT: Sorry I did not realize the age of the question.
Answer to: excessive noise from coolers after replacing hddThere is software called HDD Fan Control designed to fix this problem, it uses SMART to track the drives temperature instead of the sensor, and does control the fan correctly www.hddfancontrol.com Works great
Answer to: HDD Fan runs at 5100rpm after new 2Tb installHi David. There is software available specifically for this problem. its HDD Fan Control it will use your HDD S.M.A.R.T data to monitor your drives temperature and set the HDD Fan speed appropriately. Its just been updated to version 2 and has support for more SSD's and multiple HDD SSD setup's, and also has a nice little monitoring app so you can keep an eye on your drives temp and its fan speed. Im using it after installing a 2TB drive and its working great, I'm going to install a SSD in the optical bay this weekend and it should still work then! Well worth the money!