The latest iMacs use the sensors inside the HD itself. If you fit a different model or make the sensor can't be read and the fans go on full speed. From 2009 on Apple used the internal sensors. You could still work around the problem by using the exact same model of HD. This year (2011) things got much worse - Apple uses HDs with a proprearitary firmware that uses 7 instead of 4 pins (they actually use non standart SATA connectors) on the SATA bus for themp reading. You therefor can only use Apple branded drives for a premium pricetag. More information here.

Shame on Apple - lets work on a solution!

Here's an excellent article on upgrading your 2009. It also has links to compatible drives: http://www.everymac.com/systems/apple/im...

The problem with changing drives is the thermal sensor. If you stay with the same "brand" of drive there should be no problem. If you change brands, you will need to change to that companies sensor. As far as I know there are three different ones, the Western Digital 922-9225, the Seagate 922-9224 and the Hitachi 922-9223

Depends on the model of iMac, the newest 2011 models use a custom harddrive now that will run the fan 100% no matter what you do. On other iMacs there is a temperature sensor. Maybe this sensor was not reattached to the harddrive and is giving a false reading now.

Yours definitely have the proprietary HDD sensor if it goes crazy like this

There are a few fixes that do not involve using compatible or flashed drives to work

Look for SMART control software that is free and install that, and have it run at startup or if it gives you the option to run at startup, use it and see if that works

I have also heard the sensor is part of the power cable, matching the part number for the right cable, this has been reported to work with friends who tried it

These are general tips, but the year the mac is made will also play a major role in their effectiveness

As per age of each mac, the 2009 ones are the easiest to tame ones, SMART control software works on them, and the power cord swap may work too and is more likely, and matching drive brands could work as well, and drive brand matching too

2010 ones are something I am not entirley sure about, if I figure it out, I'll update this

2011 is probably hopeless but the power cord swap and SMART software, these are the worst ones due the HDD power connector on the drive, if this fails to fix it, game over

As to places where to get compatible drives, OWC sells some, I have been told they are compatible in some way, but cannot verify this as I own a MacBook Pro, but they do sell Apple ROM ones too and have been told these work, just be sure to get a seagate or WD and match the brand to be safe

http://eshop.macsales.com/shop/hard-driv...

OWC has a upgrade program for these Macs, but this is only for the 2010/2011 ones, these are 21.5 and 27''

http://eshop.macsales.com/shop/turnkey/i...

This is all I know about this issue with the drives

Here is a OWC blog post if you want to read it

http://blog.macsales.com/10146-apple-fur...

http://blog.macsales.com/10206-further-e...