Re-install MacOS

Booting from USB stick

After you turn on or restart your MAC, hold down the Option key immediately.

When you see the Startup Manager window, release the Option key.

Select your startup disk, and then tap the arrow or press the Return key.

Fix a problem trying to reinstall Mac OS X when installing the system

Power off, unplug the power supply after 30 seconds, option+command+r+p simultaneously press and hold the restart 5 times after installation again. (Make sure there is no problem with mirroring)

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Re-install MacOS