Hi, 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.

Wow... I have to second Harry's advice above about the iMac HDD Fan Control application. After taking my 21.5" iMac apart twice and making sure I replaced all the connections, I was ready to go back and install the original drive because I just couldn't tolerate the hard drive fan running at 5200 rpm all the time. My wife could hear it in the room next door.

Then I read about the temperature sensor on the Apple drive (doh)... but this application is golden. For $30 it saved me even having to open my iMac back up. And there's a DEMO! Now that's instant gratification for you. I didn't suspect this was a problem that could be solved by software. Thankfully I was wrong.

Funnily I had the same problem.

My solution was pretty easy - in the end. I was far too cautious in placing the RAM into the Mac. So the connection was not 100% ok. Just taking them out & plug them in.

For me this did the job completely.

In case it won't do for you; it's just 5 minutes of work ^^

Klaus

You rock Harry. I replaced the drive in my mom's iMac (17" Intel) a while back and it was fine at first but appears the sensor failed or lost connection resulting in full speed fans. I couldn't find the problem for the life of me until I came here.

Thanks to HDDFanControl it's back to normal while still protecting the machine. If you just defeat the sensor you're losing protection, HDDFanControl uses SMART to get the drive temperature and keep your machine protected. Plus you get to demo it for an hour to see if that solves you're problem or if there's more to it than that, couldn't ask for a more elegant solution.

30$ is just too much. A fix from Apple would be nice.

I'll bet it works great, but $30 is way, way too much for this. If it was $5, I would have snapped it up in a heartbeat, but not 6x that. No way.