Are your iMac's fans blaring at full speed after having upgraded or replaced its hard drive? If so then you may have overlooked a simple detail or two upon reassembling your system.
In order to replace an iMac's hard drive, you have to open the front of the system by using special tools to lift off the glass, then get underneath a number of components to reveal the hard drive bays. While not a trivial task, it is definitely doable with the proper know-how, such as following some of the guides available at iFixIt and other sources. However, some people who have done this have found their iMacs no longer give a relatively noiseless computing experience, and end up blaring their cooling fans at the highest speeds.
Since the fan speeds and temperature feedback are managed by the Mac's system management controller (SMC), if this happens, the first course of action may be to attempt an SMC reset; however, this may not fix the problem. Some have speculated that the iMac may require special firmware from Apple in its hard drive to prevent the cooling fans from being maxed out; however, this may also not be the issue at hand.
Recently the Mac-centric retailer and supply company "Other World Computing" outlined a potential issue where people upgrading their iMac's hard drives may simply have incorrectly reassembled a temperature feedback control when reassembling their systems.
In order to get to the drive, one of the steps in disassembly is to remove a temperature sensor cable that provides feedback if the LCD display gets too hot. This detector has relatively sensitive connections, so even if properly seated in its socket during reassembly, it may have an improper connection that results in the fan blaring at high speeds since no feedback is present.
Another issue that Other World Computing outlines, is where the temperature feedback for the new hard drive itself may be improperly connected. For the 2009 and 2010 iMac systems, the Western Digital hard drives Apple used have swapped connections for the temperature feedback control, so while you may have seated the sensor connection into the same socket of the hard drive according to the instructions of a reputable guide or two, you may simply have swapped the poles of this connection.
To fix either of these two issues, you will have to open the system again, and then reseat the LCD temperature sensor or flip the temperature sensor connected to your hard drive.
Other World Computing has a detailed look at the proper orientation for these connections, so if you have an iMac (and especially a 2009 or 2010 model) that has blaring fans after a hard drive upgrade, then be sure to check out Other World Computing's article to see if you fell victim to this simple oversight.