Corrupt Extensions

Extensions, like preferences, can become corrupt. Commonly, these show up as a crash or hang while the machine is starting up. Watch the icons that appear as your Mac starts up, and see if it consistently hangs at the same point. From watching which was the last icon to appear before a hang or start-up crash, you can often get an idea of which extension is the culprit.

To start up with extensions off, hold down the Shift key while starting up. If your Mac starts up with extensions off, there is a very good chance that the problem is with the extensions.

Actually, it is more common for an extension to conflict with another, than it is for an extension to become corrupt and to crash your machine.

If you have recently installed new software, check to see if any new extensions were installed. Disable, or remove them from the System folder and try to restart. If the Mac starts up normally, disable or remove all of the older extensions and enable or replace in the System folder the new extension alone. If the extension crashes or hangs you Mac when starting up without any other item, either it is corrupt or incompatible with your Mac. If it does not, it probably is conflicting with another extension.