Having lab full of 2010 and 2011 iMacs has been pretty uneventful… until last month when their 1TB Seagate hard drives started failing. One after another, a few the machines would start acting weird and freeze up. AST showed some of the drives were bad and the others I figured just hadn’t hit the error threshold yet, so I ordered 7 1TB Western Digital drives. I choose WD drives because I am in the middle of replacing 35 3TB Seagate drives under an Apple replacement program, and I have had to replace a bunch of Seagate drives in our server storage nodes, so I don’t trust Seagate to make a quality product anymore.

I went to replace two of the older machines’ hard drives, only to find instead of an external temperature sensor as the the past, Apple was attaching a single pair of wire to specific jumpers of the drive. Come to find out, the Apple hard drives have their own special firmware that even if I replaced them with the exact same model hard drives they would not be able to read the temperature and the system fans would be running full blast and AST tests would fail.

So I hit google up and found out that I could put a simple 2N3904 transistor in place of the jumper. I ordered some heat shrink and a dozen 2N3904 transistors. Looking at the technical docs for the transistor, I wired the gray wire from the iMac to the emitter and the black wire to both the base and collector pins on the transistor.

With the sensor in place (under the red tape above), the iMacs boot up without problems, the fans run normally, and AST shows a nice green check next to the sensor test.