Safe Boot (a term often used interchangeably with Safe Mode) 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 only the components that are required, Safe Boot can help you troubleshoot problems by isolating the issues. Safe Boot can get your Mac running again when you’re having problems caused by corrupt apps or data, software installation issues, damaged fonts, or preference files. In these cases, the problem you experience is either a Mac that fails to boot completely 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 these terms bandied about. Technically, they’re not interchangeable, although most people aren’t going to care which term you use. However, 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 does the following:
Performs a directory check of your startup drive Loads only the bare minimum of kernel extensions that macOS or OS X needs to run Disables all fonts other than those located at /System/Library/Fonts. These are the fonts supplied by Apple. All third-party fonts are disabled. Moves all font caches to the trash Disables all startup or login items Deletes the dynamic loader cache (OS X 10.5.6 or later) to fix problems that cause a blue screen freeze at startup
Some Features Aren’t Available in Safe Mode
Once the Safe Boot is complete, and you’re at the Mac desktop, you are operating in Safe Mode. Not all OS X features operate in this mode. Specifically, the following capabilities are either limited or don’t work at all.
DVD Player doesn’t work. iMovie can’t capture video. Devices connected to audio in or audio out don’t work. Internal or external modems don’t operate. AirPort cards may not function, depending 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 is disabled in OS X 10.6 and later.
How to Initiate a Safe Boot and Run in Safe Mode
The method you use to perform a safe boot on your Mac varies slightly depending on whether you use a wired or wireless keyboard.
Safe Boot With a Wired Keyboard
If you use a wired keyboard with your Mac, here’s how to initiate a safe boot:
Safe Boot With a Bluetooth Keyboard
The process is almost the same if you use a Bluetooth keyboard with your Mac: 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, launching Disk First Aid, or 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 update system files that may be corrupt or missing while leaving all 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.
Safe Boot vs. Secure Boot
Safe Boot is not the same as Apple’s Secure Boot, which is available for Macs released in late 2018 to the present that include the Apple T2 Security Chip. Secure Boot offers three security levels that are designed to make sure your Mac can only be started from a trusted operating system. It is not intended to replace Safe Boot.
