Bootable USB thumb drive formatted with a GUID partition type and containing an OS X installer or a usable operating system.Starting up your Mac from an external disk requires the following: This guide provides 2 ways to boot a Mac from a USB flash drive. For example, if you install macOS or Microsoft Windows on a USB drive, your Mac can recognize that drive as a startup disk. By default, Mac starts from its built-in hard disk, but a startup disk can be any storage device that contains bootable contents that compatible with your Mac.
