Let’s start with OS X. A lot of apps originally developed for 10.5 Leopard or 10.6 Snow Leopard won’t run on later versions. Snow Leopard was also the last version of OS X to support PowerPC processors. So, if you have any software that originally ran on a G3, G4 or G5 Mac, that’s as far as it could be upgraded.
Safe mode prevents your Mac from loading certain software as it starts up, including login items, system extensions not required by macOS, and fonts not installed by macOS. It also does a basic check of your startup disk, similar to using First Aid in Disk Utility. And it deletes some system caches, including font caches and the kernel cache, which are automatically created again as needed.
Safe mode is particularly useful when you're trying to resolve an issue that doesn't seem to be associated with using any particular app. As always, keeping your software up to date is the first step toward resolving any software issue.
How to use safe mode
- Turn on or restart your Mac, then immediately press and hold the Shift key as your Mac starts up.
- Release the key when you see the login window. ”Safe Boot” should appear in the upper-right corner.
- Log in to your Mac.
If the issue continues in safe mode
If the issue continues while in safe mode, or your Mac restarts several times and then shuts down while starting up in safe mode, you should reinstall macOS and make sure that your Apple software and third-party apps are up to date.
If the issue doesn't continue in safe mode
If the issue doesn't continue while in safe mode, leave safe mode by restarting your Mac normally. Then test for the issue again. The issue might be resolved.
If the issue returns after leaving safe mode, startup items are probably at fault. Apps can install such items in numerous places, and they aren't always easy to find or recognize. For that reason, you should first make sure that you've installed any available updates for your Apple software and third-party apps.
If updating your software doesn't help, try any of these solutions:
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- Unplug all accessories from your Mac, including printers, drives, USB hubs, and other nonessential devices. You could have an issue with one or more of those devices or their cables.
If you still need help, please contact Apple Support.
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Some features might not work in safe mode, such as movie playback in DVD Player, video capture, file sharing, Wi-Fi, some accessibility features, some audio devices, and some devices that connect via USB, Thunderbolt, or FireWire. Accelerated graphics are disabled, which could cause blinking or distortion onscreen during startup.