I’ve long recommended creating a bootable installer drive–on an external hard drive, thumb drive, or USB stick–for the version of OS X you’re running on your Mac.1 It’s great for installing the OS on multiple Macs, because you don’t have to download the ~5GB installer onto each computer, and it serves as a handy emergency disk if your Mac is experiencing problems. 2 Here’s this year’s version, for macOS Sierra (formerly known as OS X 10.12), of my annual how-to guide.
Dan Frakes updates this article annually, and it’s a must-read for all Mac users.
Source: How to make a bootable macOS Sierra installer drive | DanFrakes.com