If your Mac starts up to a circle with a line through it - Apple Support

A circle with a line through it means that your startup disk contains a Mac operating system, but it's not a macOS that your Mac can use. 

Mac Boots to A Circle with Line Through it 🚫, How to Fix?

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If your Mac starts up to a circle with a line through it - Apple

How To Fix Mac Boots to a Circle with Line Through it 🚫

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