macOS Sierra (10.12), the first of the new macOS systems, offers the ability to create a bootable installer on a USB flash drive or on a drive you connect to your Mac.

A macOS Sierra bootable installer lets you perform a clean installation, completely replacing the contents of your Mac’s startup disk with a brand new, fresh installation of Sierra. The bootable installer can also be used to install macOS Sierra on multiple Macs without having to download the installer app from the Mac App Store each time. This is a useful feature if you have a problematic or slow internet connection.

Install media for OS X and macOS have been available for a while, but they’re not often used. The command to create the bootable installer is hidden in the installer you download from the Mac App Store. That installer download will start automatically once the download is complete. Clicking the Install button will automatically remove the downloaded installer as part of the normal installation process, making it impossible to use it to create a bootable macOS Sierra installer yourself.

Before you begin the process of creating a bootable installer, you may need to perform some housekeeping tasks. Creating the bootable installer requires formatting the bootable media (a flash drive or external drive), which will result in erasing any data the destination volume may contain.