Original post by: Lars De Richter ,


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).

# Hold option key while booting
# Select the correct startup disk
# Hold control key while tapping Enter to make it stick (the icon above the disk should become a circle)