4 
3 Spinning gear on start-up freezes and does not boot... to anything.
So, my iMac won't boot. Here are the things I have done:
- PRAM
- "Disconnect-from-power-hold-power-button-down"
- Safe-boot
- single user
- hardware test
- boot from install dvd
... I think that is it.
When ever I try to boot to the HD or an install disk or an install DVD from a external HD disk image, the typical loading screen appears with the apple logo and the spinning gear. after about 20 seconds the gear freezes and that is all.
After doing some research I think that it may be the graphics card but want to get another opinion. I will also be dropping it off at the apple store in the near future to see what they say.
The Hardware test came back and said that everything appears to be ok with no errors.
Update
So I dropped off the iMac late last week. The genius ran all the testes on it and couldn't determine the issue so I left it there for them to screw with. Got a call from apple today saying that the hard drive wasn't allowing the system to boot, which is interesting because I tried to boot from an external drive and when that didn't work I used another iMac to install a new OS on the drive while it was in target disk mode. I was then able to to boot using that hard drive with the functioning iMac but not with the broken iMac. I will go pick it up and remind them of this issue and double check with them to make sure that they actually pulled out the old drive and stuck in a new one to make it work. craziness. I will keep you posted.
Is this a good question?
1 
I am confused about how the machine won't boot to an install disk, yet you were able to run AHT. It had to boot to run Apple Hardware Tests (AHT). Is it booting to the install disk and you are not able to install? Have you changed anything in the system recently? Did you just purchase this machine in this condition? What are the diagnostic LEDs saying is going on? Those LEDs will tell you if there is a problem with your GPU (video chip). Try reseating your RAM and doing an SMC reset. Google the reset if you are not familiar with it, for instructions. Please let us know the answers to these questions and the results of the suggestion(s).
by ABCellars
I know, weird huh? I can boot to the disk selection screen just fine by holding the option button but when I select the install disk it starts loading the data and then stops... and the wheel stops. but when I boot directly to the AHT, it does just fine. I purchased the computer recently, I haven't had this issue before and it sounded like a corrupt OS but obviously it not directly related to that. I have located the SIL's on an older machine before but I am not sure where they are on this model. Anyone have any pictures?
I am bringing it to apple tomorrow just to see what they say, a machine shouldn't be totally screwed after 4 years right? I haven't had opportunity to reseat RAM yet but will tonight.
by Shawn