Learn how to boot Apple devices into Safe Mode. Safe Mode allows you to safely power on your computer in the event that there is a system issue, and can help resolve various problems with your Mac. This document will explain the various reasons you may wish to boot your Apple computer into safe mode, as well as the steps on how to do so.
Starting up into Safe Mode on your Mac computer does several things:
Forces a directory check of the startup volume. You may see a progress bar on the screen during this check, and the computer may take longer than usual to complete its startup process
Loads only required kernel extensions (some of the items in /System/Library/Extensions)
In Mac OS X v10.3.9 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)
Disables all fonts other than those in /System/Library/Fonts (Mac OS X v10.4 or later)
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 v10.4 or later)
Disables all startup items and login items (Mac OS X v10.4 or later)
Mac OS X v10.5.6 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.