A common mac service we provide is the replacement of internal hard disk drives, whether the user needs more drive capacity or there is a fault with the drive.
Since late 2009, replacing a hard disk on an iMac has caused the fans to spin up to 6000 RPM, which is noticeably loud. Apple have been replacing the external hard drive sensor with a proprietary firmware and using the drive's internal sensor.
When you open an iMac you will find a cable attached to the back of the hard disk in addition to the regular power and sata cable connector... looks like this:
Replacement drives just do not provide the iMac with the appropriate information it needs about the drive temperature, so the system assumes there is a problem and speeds up the fans... resetting SMC has a temporary effect.
Some people short the temperature sensor, but this will mean the system cannot protect itself in the event that the drive is in fact overheating.... bad.
We've purchased a piece of software called HDD Fan Control. On installation, it starts reading the drive SMART information, and can monitor an accurate reading of the drive temperature. It then in turn sets the fans at the appropriate temperature...
No more fan noise... and no overheating!!! If you want some assistance with this or any other mac service query, let us know!