macOS: Boot failed after update installation

My Mac refused today to boot because of this message:

macOS could not be installed on your computer

The path /System/Installation/Packages/OSInstall.mpkg appears to be missing or damaged. Quit the installer to restart your computer and try again.

After several restarts the problem persisted and the update won't install correctly 🤨

Solution

To persuade your Mac to boot again correct, just press the start the options (or alt) key during startup.

There should be displayed at least two boot drives:

After a click on "Macintosh HD" the OS should load normal and will be useable again 🙂

To trigger an re-download of the failed update, just enter this command inside a shell:

sudo softwareupdate --install --all