Part of your “backup plan” for a workstation must include a way to boot from an external source. And this is no different on Mac, regardless of the marketings Apple is selling. Although it is possible to boot a Mac with internet recovery mode, taking bets on at-risk computers are not advisable.

Any of the following options would require at least 12GB of storage according to Apple’s literature. I would recommend simply buying 16GB USB drive dedicated for this particular job as a backup. Each major update has different path as well, so before you go ahead, make sure you are creating a bootable disk of the desired version.

1. Before we do anything on the drive, make sure to wipe the drive. Open up Disk Utility, click on the drive from the sidebar, and click erase.
2. Now the drive should appear on the desktop with the name you had given to it. Default value is Untitled.
3. Open up the Terminal, and copy-paste one of the following codes. Replace the bracketed part with the path to the desired volume, by simply dragging the volume icon on the Desktop to the Terminal window.

   High Sierra:
    sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume [Volume Path]
   

   Sierra:
    sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume [Volume Path] --applicationpath /Applications/Install\ macOS\ Sierra.app
   

   El Capitan:
    sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app
   

   Yosemite:
    sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume [Volume Path] --applicationpath /Applications/Install\ OS\ X\ Yosemite.app
   

   Mavericks:
    sudo /Applications/Install\ OS\ X\ Mavericks.app/Contents/Resources/createinstallmedia --volume [Volume Path] --applicationpath /Applications/Install\ OS\ X\ Mavericks.app
   

4. (optional) If you would like to be notified when will it be done, simply add the following at the end of the code. It will make audible announcement when the copying is complete. Make sure to separate the existing code and the optional one with a space.

   [Existing code] && say 'disk done'
   

5. Run the code you have entered to the terminal so far. Process can take 10 to 20 minutes.

I can’t stress this enough, but try to keep a bootable USB handy with the most recent version of OS, at the very least up to the most recent major release. (i.e. keep a copy of 10.13, if 10.13 is the most recent major release) Also if you happen to do regular fresh-install on your computers, I highly recommend using Time Machine. It can come in very handy to restore important files after a fresh install.