How to Use Your Mac's Safe Boot Option

Check your drive and clear most system caches

Apple has offered a Safe Boot (sometimes called Safe Mode) option ever since Jaguar (OS X 10.2.x). Safe Boot can be a key troubleshooting step when you're having problems with your Mac. These can be problems with starting your Mac up or issues you come across while using your Mac, such as having apps not start or apps that seem to cause your Mac to freeze, crash, or shutdown.

Safe Boot works by allowing your Mac to start up with the minimal number of system extensions, preferences, and fonts it needs to run. By minimizing the startup process to just those components that are required, Safe Boot can help you troubleshoot problems by isolating the issues.

Macbook displaying Safe Boot

Safe Boot can get your Mac running again when you're having problems caused by corrupt apps or data, software installation issues, or damaged fonts or preference files. In all cases, the problem you may experience is either a Mac that fails to completely boot and freezes at some point along the way to the desktop, or a Mac that boots successfully, but then freezes or crashes when you undertake specific tasks or use specific applications.

Safe Boot and Safe Mode

You may have heard both of these terms bandied about. Technically, they're not interchangeable, although most people aren't going to care which term you use. But just to clear things up, Safe Boot is the process of forcing your Mac to start up using the bare minimum of system resources. Safe Mode is the mode your Mac operates in once it completes a Safe Boot.

What Happens During a Safe Boot?

During the startup process, a Safe Boot will do the following:

  • Perform a directory check of your startup drive.
  • Load only the bare minimum of kernel extensions OS X needs to run.
  • Disable all fonts other than those located at /System/Library/Fonts. These are the fonts supplied by Apple; all third-party fonts will be disabled.
  • Move all font caches to the trash.
  • Disable all startup or login items.
  • Delete the dynamic loader cache (OS X 10.5.6 or later). This can fix problems that cause a blue screen freeze at startup.

Some Features Won't Be Available

Once the Safe Boot is complete, and you're at the Mac desktop, you'll be operating in Safe Mode. Not all OS X features operate in this special mode. Specifically, the following capabilities will either be limited or won't work at all.

  • DVD Player won't work.
  • iMovie won't be able to capture video.
  • Devices connected to the audio in or audio out won't work.
  • Internal or external modems won't operate.
  • AirPort cards may not function. This depends on which version of the card and which version of the OS is in use.
  • Quartz Extreme won't run. Applications that use Quartz Extreme features, such as translucent windows, may not work correctly.
  • Network file sharing will be disabled in OS X 10.6 and later.

How to Initiate a Safe Boot and Run in Safe Mode

To Safe Boot your Mac with a wired keyboard, do the following:

  1. Shut down your Mac.

  2. Press and hold the shift key.

  3. Start up your Mac.

  4. Release the shift key once you see the login window or the desktop.

To Safe Boot your Mac with a Bluetooth keyboard, do the following:

  1. Shut down your Mac.

  2. Start your Mac up.

  3. When you hear the Macs startup sound, press and hold the shift key.

  4. Release the shift key once you see the login window or the desktop.

With your Mac running in Safe Mode, you can troubleshoot the issue you were having, such as by deleting an application that's causing problems, removing a startup or login item that's causing issues, or launching Disk First Aid and repairing permissions.

You can also use Safe Mode to initiate a reinstall of the current version of the Mac OS using a combo update. Combo updates will update system files that may be corrupt or missing while leaving all of your user data untouched.

In addition, you can use the Safe Boot process as a simple Mac maintenance procedure, flushing many of the cache files the system uses, preventing them from becoming too big and slowing some processes down.