If you were using El Capitan, you probably have to insert the following command in the terminal: csrutil disable And on a Macbook Air it was a bit complicated to choose an external startup disk and make it stick (otherwise you get back to the same situation). (1) Hold option key while booting (2) Select the correct startup disk (3) Hold control key while tapping Enter to make it stick (the icon above the disk should become a circle)
