iMac boot problem fix

There are a few of ways to solve boot problems such as this:

Remove peripheral hardware

If you have USB or Firewire devices attached to your system, unplug them and try booting without them attached. Holding down the shift key may also help prevent unnecessary kernel extensions from loading. A number of people with this problem have found that certain devices plugged into their USB ports seem to have been contributing to this problem.

Clear boot caches

Use a system cleaning program such as OnyX, MainMenu, or MacCleanse to empty the boot cache, forcing the system to refresh it with the drivers necessary for system use. The first reboot after doing this will take longer as the system finds and loads the necessary drivers into the new cache.

Reset PRAM and SMC

The PRAM and SMC contain a number of boot arguments and settings for the system hardware, which may contribute to boot hangs if they are not set correctly. Try resetting the PRAM and SMC for your system, and see if the problem goes away.

Resetting the PRAM

Restart the computer with the options-command-P-R keys held down. The system will continually reset and make the boot chimes while these keys are held. Let it to cycle a couple of times and then release the keys and allow it to boot normally.

Resetting the SMC

The system management controller can be reset for iMacs by turning off the computer and unplugging all connections from it for at least 15 seconds, as per the instructions in this Apple Intel-based iMac, Intel-based Mac mini: How to reset the System Management Controller article

Try different peripheral connections

If the system boots fine with no peripherals attached, try attaching them one at a time and rebooting to troubleshoot which one is causing the problem. It may be that a specific USB or Firewire port configuration is to blame, so when you find the device that's causing the problem, try putting it in a different port and try again. This is particularly true if you have devices daisy-chained together, which can sometimes lead to device conflicts or power issues.