How to make a bootable High Sierra installer drive in case you want to downgrade from the macOS Mojave beta

The macOS Mojave public beta is on everyone’s mind, but you should keep a copy of macOS High Sierra, too.

If you are planning to install the macOS Mojave public beta, there is one additional set of steps you should follow. You should download a bootable copy of macOS High Sierra. This allows you to use it if you partition your hard drive for dual-software installation. It is also the only way to downgrade to the older format if you decide you don’t like the macOS Mojave beta at all.

Before you start

Before you get started, make sure you have a thumb drive with at least 8GB of storage, or a spare external hard drive (one that you don’t use with anything else).

You’ll also need to download macOS High Sierra from the Mac App Store.

Note: After macOS High Sierra has downloaded, it will automatically begin the installation process. Close the installer instead.

Please, Please, Please, backup your Mac before you do anything.

How to format your external drive for Mac

You’ll need to start with a clean thumb drive or hard drive in order to get it ready to work as a bootable drive.

  1. Plug the USB end of the drive to a USB port on your Mac.
  2. Click on Finder in your Dock to open a Finder window.
  3. Select Applications from the list on the left side of the window.

  4. Scroll down and double-click on Utilities.
  5. Scroll down and double-click on Disk Utility.

  6. Select your drive under External.
  7. Click on the Erase tab at the top of the window.

  8. Note the name of your external hard drive (probably “Untitled”) because you will need it when you create a bootable drive. If you have more than one external hard drive named “Untitled,” you will need to rename your hard drive now.
  9. Select Mac OS Extended (Journaled) from the format list.
  10. Click Erase.
  11. Click Done when the process is complete.
  12. Close the Disk Utility window.

Your thumb drive or external hard drive is now ready.

How to make your external drive bootable for installing macOS High Sierra

Important:You will need to be in your Mac’s administrator account in order to run the sudo Terminal command.

  1. Click on Finder in your Dock to open a Finder window.
  2. Select Applications from the list on the left side of the window.

  3. Scroll down and double-click on Utilities.
  4. Scroll down and double click on Terminal.

  5. Recall the name of your formatted external drivewhen entering the following text into Terminal. If it is not named “Untitled,” you will need to change the command syntax for the pathname where it says: Volumes/Untitled. The name of the drive can’t have any spaces and it is case sensitive.
  6. Enter the following text into Terminal (Don’t forget to change the name “Untitled” if your hard drive is named something else):

    sudo /Applications/Install macOS High --volume /Volumes/Untitled --applicationpath /Applications/Install macOS High

  7. Hit the enter/return key.

  8. Enter your administrator account password. This is the password you use to make changes on your Mac or log in. No text will appear in Terminal when you enter the password.

The process could take a very long time, depending on the drive. When it is done, the Terminal window will report “Done.”

How to use macOS High Sierra with an installer boot drive

Once macOS High Sierra is installed on your external drive, you can boot up and install any Mac with the drive plugged into it.

  1. Turn off the Mac you want to install macOS High Sierra with the bootable drive.
  2. Connect the external drive to your Mac via the USB port.
  3. Turn on your Mac.
  4. Hold down the Option key when you hear the startup chime.
  5. Select the external drive with macOS High Sierra on it from the list of systems to start up your computer.

The installation process will begin. Follow the steps to install macOS High Sierra onto your Mac or partitioned hard drive.

Do you have any questions about how to create a bootable installer drive for macOS High Sierra? Let us know in the comments.

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