If you are planning to try out the beta of macOS Catalina, there is one additional set of steps you should follow. You should download a bootable copy of macOS Mojave. This allows you to have a portable installer for putting it on multiple Macs and it is also the only way to downgrade if you decide you don't like the beta of macOS Catalina.

Before you start

Before you get started, make sure you have a thumb drive with at least 15GB of storage, or a spare external hard drive (one that you don't use with anything else).

You'll also need to download macOS Mojave from the Mac App Store.

Note: After macOS Mojave 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.

    Opening Finder on Mac

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

    Selecting Disk Utility on Mac

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

    Erasing a drive on Mac

  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. If your Mac is using HFS+, select Mac OS Extended (Journaled) from the format list. If your Mac is using APFS, select APFS from the format list.
  10. If Scheme is available, select GUID Partition Map.
  11. Click Erase.
  12. Click Done when the process is complete.
  13. Close the Disk Utility window.

    Confirm erase thumb drive

Your thumb drive or external hard drive is now ready.

How to make your external drive bootable for installing macOS Mojave

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.

    Opening Finder on Mac

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

    Opening Terminal on Mac

  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\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled

  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.
  9. Type Y to confirm that you want to erase the drive and press return.

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

The Terminal screen on Mac

How to use macOS Mojave as an installer boot drive

Once macOS Mojave 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 Mojave 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 Mojave on it from the list of systems to start up your computer.

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

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

Updated June 2019: Updated for the public beta of macOS Catalina.

Some things you may need for the public beta of macOS

Samsung T5 SSD (From $85 at Amazon)

Samsung's T5 SSD has super fast transfer rates with write speeds up to 540MB/s, which is great for backing up your Mac before installing a beta.

Belkin Thunderbolt 3 cable ($30 at Amazon)

For up to four times transfer speed, make sure you're using a Thunderbolt 3 cable. Belkin's brand supports up to 100W power delivery and measures 1.6-feet long for a little extra length.

SanDisk Dual Drive (From $25 at Apple)

If you want to make a bootable installer of a new macOS or just want to preserve an older macOS in case you want to downgrade, SanDisk's Dual drive is perfect. It has a reversible USB-C and USB-A connector, too!

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