To avoid this you should do a regular disk check (as I should have been doing):

- Restart your Mac - Hold down Apple key + S straightaway - You will eventually see it boot into Unix (black screen) - When it has finished type the following command:

fsck -fy

Press Enter and let it do its stuff. This could take awhile so go make a coffee or something.

When it has finished type the following command:

reboot

And press Enter. Computer will reboot.

If this won’t work for you can use the following alternative command:

fsck -f

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