folder with a ? When boot up a computer

In case the user missed the information in supplied link from previous poster

If your Mac starts up to a question mark

A folder with a flashing question mark means that your startup disk is no longer available or doesn't contain a working Mac operating system.

If the question mark is persistent, and your Mac does not start up normally:

1. Press and hold the power button on your Mac for up to 10 seconds, until your Mac turns off.
2. Every Mac has a power button. On notebook computers that have Touch ID, press and hold Touch ID.
3. Use Disk Utility to repair your startup disk. Follow the steps for a Mac with an Intel processor.
4. If Disk Utility found no errors or repaired all errors, reinstall macOS.
5. If you still need help, please contact Apple Support.

Click here and follow the instructions.

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