In case something happened to your Mac and you can’t get it to start, you just plug in your bootable USB drive (or any other drive), wait for your Mac to detect it, then instruct your Mac to use the bootable USB drive to load your operating system instead.
This is when you use a bootable USB drive.Bootable drives can be any type of storage device (USB drive, CD, DVD, external hard drive, etc) that contains a copy of your operating system. By default, your Mac boots from your internal hard drive, but, if for one reason or another, your hard drive won’t boot, your Mac will ask you if you want to use a different drive instead. Whenever your Mac starts, it initiates what is called a boot process where it tries to load your operating system so that you can use your computer. A good way to do this is by creating a bootable USB drive. While the option is no doubt more convenient than keeping physical copies of your Mac’s operating system around, though, keeping a backup copy of your operating system is still a good idea in case something happened to your computer and it refused to start. Installing or reinstalling operating systems has been a lot easier since Apple decided to stop shipping installation discs and offering them as downloadable files from the Mac App Store instead.
