This article is adapted from an Apple knowledge base article at http://support.apple.com/kb/HT1564?viewlocale=en_US&locale=en_US

Starting up in Safe Mode does several things:

  • It forces a check of the startup volume, just like the First Aid feature of Disk Utility. You may see a progress bar on the screen during this check, and the computer takes longer than usual to complete its startup.
  • It loads only required kernel extensions.
  • It disables all fonts installed by the user.
  • It moves font caches to the Trash that are stored in /Library/Caches/com.apple.ATS/(uid)/  – where (uid) is a user ID number such as 501 (Mac OS X v10.4 or later).
  • It disables all startup items and login items in Mac OS X v10.4 or later.
  • In Mac OS X v10.3.9 or earlier, Safe Mode opens only Apple-installed startup items (such items may be installed either in /Library/StartupItems or in /System/Library/StartupItems). These items are different from user-selected account login items.
  • Rebuilds the Launch database stored on the hard disk.

Taken together, these changes can help resolve or isolate certain issues that exist on the startup volume.

Starting up in Safe Mode

To start up into Safe Mode (to Safe Boot), follow these steps.

  1. Be sure your Mac is shut down.
  2. Press the power button.
  3. Immediately after you hear the startup tone, hold the Shift key.
    The Shift key should be held as soon as possible after the startup tone, but not before the tone.
  4. Release the Shift key when you see the progress bar below the gray apple. (See the picture below.) OS X versions 10.5.8 and earlier don’t display the progress bar, so the safest thing to do is continue holding the Shift key until you get to the login screen.

Safeboot

To leave Safe Mode, restart your computer without holding any keys during startup.