When swapping the HDD/SSD in a 2009-2010 iMac, the HDD fan will go haywire if the HDD is not an Apple OEM drive, or some trick is done with the thermal sensor such as shorting its pins, replacing it with a transistor or use a solution like OWC's inline thermal sensor solution(s) (they make several variations for different Macs). Below are some images of two such solutions for reference, if you've never seen them before.
OWC's thermal sensor solution for the 2009-2010 iMac - a small PCB with a couple of surface mount components, powered by SATA power:
Original 922-9224 Seagate thermal sensor cable that plugs into the HDD (top), ODD thermal sensor from 2008 iMac (bottom) - looks a lot like a bipolar transistor soldered to a couple of wires:
The OWC looks more complex; it's powered and has a bunch of stuff on it, but the ODD thermal sensor approach seems to achieve the same outcome. What is the technical difference between these four approaches:
- OWC inline thermal sensor
- Shorting the pins
- Using an optical disc drive thermal sensor made for another/an older iMac
- Chopping up the original sensor cable and soldering a bipolar transistor to it (say, the 2N3904)