How to Safe Boot into Safe Mode
If you're having difficulty starting your Mac running OS X Mountain Lion, booting your Mac in Safe Mode might help you resolve your startup issue by not loading nonessential (and non–OS X) software at boot time. You do it by holding down the Shift key during startup.
If your Mac is set up so you don’t have to log in, keep pressing the Shift key until the Finder loads completely. If you do log in to your Mac, type your password as usual, but before clicking the Log In button, press the Shift key again and hold it until the Finder loads completely.
You know you held the Shift key long enough if your Login Items don’t load (assuming that you have Login Items; you can designate them in the Users & Groups System Preferences pane, although some programs create them for you).
Booting in Safe Mode does three things to help you with troubleshooting:
It forces a directory check of the startup (boot) volume.
It loads only required kernel extensions (some of the items in /System/Library/Extensions).
It runs only Apple-installed essential startup items (some of the items in /Library/StartupItems and /System/Library/StartupItems). Note that the startup items in the Library folders are different from the Login Items in the Users & Groups System Preferences pane.
Taken together, these changes often work around issues caused by software or directory damage on the startup volume.
Some features don’t work in Safe Mode. Among them are DVD Player, capturing video (in iMovie or other video-editing software), using an AirPort card, using some audio input or output devices, or using a USB modem. Use Safe Mode only if you need to troubleshoot a startup issue.
If your Mac boots in Safe Mode, you might be able to determine what’s causing the issue by moving the contents of your Preferences folder (in Home/Library, which you can make visible by pressing the Option key when opening the Go menu) to the Desktop temporarily or by disabling Login Items (in the Users & Groups System Preferences pane).
If either of these things resolves the issue, you can put preferences files back in Home/Library/Preferences a few at a time, or you can re-enable login items one at a time until you figure out which preferences file or login item is causing your problems.