You could also use a secondary internal partition.

Mount your USB flash drive or other volume.

softwareupdate -fetch-full-installer -full-installer-version 11.0.1 Option 2: Download the macOS installer via terminal. The installer will be in your Applications folder. Quit the installer if it opens automatically after downloading. Option 1: Download the macOS installer from the Mac App Store.

There are two options for downloading the macOS installer.

Overview Use the createinstallmedia command in Terminal I personally use the w/Options command which include both the -nointeraction and &say Installation commands. Final macOS Executable Commands labled as Full Example or Full Example w/Options. The final executable command(s) are found within Section III. These advanced steps are intended primarly for system administrators and others who are familiar with the command line.

With macOS, you can use a USB flash drive or other removable media as a startup disk from which to install macOS. The following instructions were predominantly sourced via this Apple Support Document.

Make bootable usb mac os sierra