If your Mac stops or delays while starting up
If your Mac freezes, displays a flashing question mark, or delays while starting up, you may need to select a different startup disk or repair your hard disk.
Note: To print these instructions, press Command-P.
If your Mac starts up after a brief delay or displays a question mark while starting up, you may need to select a startup disk. Choose Apple menu > System Preferences, click Startup Disk, then select a disk.
Important: If you have a Mac with an Apple T2 chip, added security features may need to be set in order to change the startup disk. See What is the Startup Security Utility?.
If you see a white screen with the Apple logo and a progress bar, macOS is checking your hard disk. This check could take a few moments.
If you start up your Mac and the Apple logo doesn’t appear, press and hold the power button for about 10 seconds to shut down your Mac. Press and hold the Option key and start up your Mac again. Keep holding down the Option key until one or more disk icons appear on your screen. Select one of the disks. After your Mac starts up, choose Apple menu > System Preferences, click Startup Disk, then select a disk.
If the lock at the bottom left is locked , click it to unlock the preference pane.
If your Mac doesn’t start up or there’s a delay while starting up, disconnect all accessories, including FireWire, USB, USB-C, and Thunderbolt devices. Connect an Apple keyboard and mouse to your Mac. Do not use hubs or extension cords.
If you can use your Mac now, there may be a problem with one of the other devices. Try reconnecting the other devices, one by one, to your Mac and restarting each time after you connect the device. When you find the device that causes the problem, review its documentation for further troubleshooting steps.
Uninstall all third-party devices, such as PCI cards and ExpressCard/34 cards.
If you can use your Mac now, there may be a problem with one of the devices. Try reinstalling the devices in your Mac, one by one, and restarting each time after you install a device. When you find the upgrade that causes the problem, review its documentation for further troubleshooting steps, or contact the manufacturer.