In this article we'll run through the steps necessary to create a bootable installer of the Mac operating system, whether it's so you can install the latest version of the macOS on multiple Macs; perform a clean install of the macOS; or just to be prepared with a handy emergency disk if your Mac experiences problems and you don't want to connect to the internet.

So if you want to find out how to make a bootable installer of macOS Sierra, the beta version of macOS High Sierra, or an older version of Mac OS X, you've come to the right place.

You can also use your bootable installer to install the MacOS on a separate partition of your hard drive, it's also a hassle-free way to go back to the previous version of the MacOS.

Luckily, making a bootable installation of the Mac operating system became a whole lot easier when Apple launched Mavericks back in 2013. With that version of Mac OS X, and all versions since, it's been possible to use the Terminal command createinstallmedia to create a bootable installer of the macOS, in this article we'll show you how.

What you’ll need

A 12GB Flash Drive (at least!)

The installation file for Sierra is 5GB, but you’ll need a drive with at least 12GB storage available (it used to be 8GB, but Apple now recommends 12GB.)

We recommend a Flash memory stick if you are planning to boot from the drive as it will be quicker.  

If you have data on the drive that you plan to use you will need to transfer it to another drive, or get a new drive, as it will be completly formatted.

The MacOS installation files

You will need to get the installation files from the Mac App Store. If you already have the software installed on your Mac you will find it in the Mac App Store under your purchased tab.

It goes without saying that you will need an internet connection to download the software.

Here's how to get the MacOS Sierra installation files (the same process applies if you are getting an older version of the MacOS, but you may need to look under your Purchased tab):

  1. Launch the App Store on your Mac
  2. Look for macOS Sierra in the store (if you have previously downloaded Sierra you will need to search under your Purchased tab)
  3. Click on the Download button, and your Mac will download the installer to your Applications folder
  4. If it automatically launches after download, quit - you don’t want to start the process of installing it on your Mac because doing so will delete the installers - and it’s the installers you need. If it opens, close the installer.
  5. You will find the installation files in your /Applications folder. Access it via the Finder.

How to make a bootable macOS installer

Note, the createinstallmedia method described here doesn’t work under OS X 10.6 Snow Leopard - it requires OS X 10.7 Lion or later. Also, the processes have changed slightly since Mavericks so if your looking to create an installation of one of the ‘Cat’ versions of Mac OS X you should read this instead.

Creating a bootable installation of MacOS requires a single command in Terminal. The createinstallmedia command makes it possible to create a bootable copy of an installer on any drive that’s connected to your Mac.

Note that the createinstallmedia command erases anything on your external disk though, so make sure there’s nothing on it that you need.

These are the instructions to follow if it’s the macOS Sierra installer you require, although if you wanted a different version of a post Mavericks OS, the instructions will be similar.

  1. Plug in an external drive with at least 5GB space as that’s how much the installer will take up

  2. Change the name of the drive to Untitled as this will make the steps below easier to follow

  3. Open Terminal (the easiest way is to press cmd+space bar and then start typing Terminal)

  4. Copy the following text into Terminal:

    sudo /Applications/Install\ macOS\ --volume /Volumes/Untitled --applicationpath /Applications/Install\ macOS\
  5. Click Enter
  6. Terminal will ask for a password. This is your user password. Note you won’t see characters appear as you type it in, that’s find. After typing in your password, press Enter.
  7. Terminal will warn that it is about to erase the drive (so make sure there wasn’t anything important on it!). If you want to continue press Y and then Return. The process can take a while, you'll see “Erasing Disk: 0%… 10%… 20%… 30%…100%… 
  8. If after erasing your drive you see a message asking if you’d like to use the drive for Time Machine be sure to click on Don’t Use.
  9. Now Terminal will spend a few minutes copying the installer file to your drive. "Copying installer files to disk… Copy complete" and so on will appear in the Terminal window.
  10. When Terminal has finished copying the installer you will see the words Copy complete and Done appear.
  11. Now you have the installer on the external drive you can use that to boot from. Plug the external drive into the Mac that you want to install the macOS on.
  12. Start up the Mac, holding down the Option/Alt key while it is booting up.
  13. Your Mac will display the Startup Manager, click on your Sierra drive and select Install OS X.

You can also run the Mac operating system directly from an external drive rather than your built-in startup disk, this is handy if you are testing new versions of the Mac OS. The process is different to the one described above though, and we cover it here: Read about How to run macOS on an external hard drive here.