Stuck on grey startup

My iMac 27" mid 2011 (OS X 10.9.1) got stuck on the grey startup screen yesterday. It shows the apple logo and the spinning gear and refuse to move on from there. My bootcamp Windows 7 on the same machine works perfectly fine and I can access all the data on my OS X system through there.

I have tried:

1. Resetting the PRAM and SMC, which did nothing except give me back the loud startup sound.

2. Disconnect all peripherals, even keyboard and mouse. No difference.

3. Safe boot, which produced the same problem except that I got a progress bar att the bottom which got to about a third before it dissapeared and the computer remained stuck.

4. Starting from the recovery disk. No difference.

5. Starting from a fresh system on a SD-card. No difference.

6. Starting via Target Disk Mode. The iMac was unable to launch Target Disk Mode, it's just the same grey apple logo with the spinning gear. When I set my macbook air in Target Disk Mode I can choose that one as startdisk for my iMac but with no change to the problem.

7. Just leaving it on the startup screen for 20 minutes. No difference.

Number 4, 5 and 6 in the above list makes me think it's a hardwareproblem, but since the bootcamp system works problem free I'm thinking it must be a very specific hardware problem.

So my first question is: is there some single piece of hardware that is so important for OS X it won't start without it, but completely unused by windows?

My second question is simply: does anyone have any suggestion how I can solve this?

//Stefan