Actually, just pressing alt or option on an external keyboard should go to the boot partition selector. If you are using a wireless keyboard, do not press the k

Actually, just pressing alt or option on an external keyboard should go to the boot partition selector. If you are using a wireless keyboard, do not press the key until you hear the startup chime, because bluetooth is not loaded until then: https://www.cnet.com/news/how-to-manage-os-x-boot-options-with-wireless-keyboards/ https://www.cnet.com/news/how-to-manage-os-x-boot-options-wi... Using Apple's BootCamp or rEfIt might also help if this does not work for some reason.