I just want to be SAFE — What is Safe Mode?

Safe mode is available in Mac OS X 10.2 or later, and is one of the more useful ways to troubleshoot the Macintosh OS. To start in Safe Mode, hold the Shift key down as your Mac starts up.

Starting up into Safe Mode does several things:

  • It forces a directory check of the startup volume.
  • It loads only required kernel extensions (some of the items in /System/Library/Extensions).
  • In Mac OS X 10.3.9 or earlier, Safe Mode runs only Apple-installed startup items (such items may be installed either in /Library/StartupItems or in /System/Library/StartupItems; these are different than user-selected account login items).
  • It disables all fonts other than those in /System/Library/Fonts (Mac OS X 10.4 or later).
  • It moves to the Trash all font caches normally stored in /Library/Caches/com.apple.ATS/(uid)/ , where (uid) is a user ID number such as 501 (Mac OS X 10.4 or later).
  • It disables all startup items and login items (Mac OS X 10.4 or later).
  • Mac OS X 10.5.6 or later: A Safe Boot deletes the dynamic loader shared cache at (/var/db/dyld/). A cache with issues may cause a blue screen on startup, particularly after a Software Update. Restarting normally recreates this cache.

Taken together, these changes can help resolve software or directory issues that may exist on the startup volume.

The Disadvantages:

A normal boot sequence may take 10-30 seconds, but when performing a safe boot all the normal caches are either destroyed or being rebuilt, so a safe mode boot may take up to 3-5 minutes.  Also some features don’t work in Safe Mode  For example, you can’t use DVD Player, capture video in iMovie, use audio input or output devices, or use an internal or external USB modem. The behavior of some devices such as an AirPort card may vary depending on computer and Mac OS X version.

Safe Mode in Mac OS X 10.5 or later also disables Quartz Extreme (hardware accelerated graphics). Applications that depend on Quartz Extreme will not open, and the Mac OS X menu bar will appear solid even if “Translucent Menu Bar” is enabled in Desktop & Screen Saver preferences.