Introduction

The following documentation and instructions are from the Apple website, knowledge-base support document number HT201372. This is the most helpful document available. This introduction has been added to specifically address Catalina.

The missing component in the instructions below, and it’s an essential one, is that you need to have the desired OS installer downloaded and available in your Applications folder on a running Apple computer. If you’re having problems with your computer, this isn’t possible. If you’re using another computer to perform this task, and it is an older computer that can’t run Catalina, you won’t be able to download the Catalina installer.

An unhelpful document (HT201475) suggests that you install Catalina from the App store, which is not possible if your computer isn’t running properly, and can’t be done on another computer unless it is new enough to be compatible with Catalina.

If you have a working Apple computer, then you probably don’t need a Catalina bootable installation media USB. If your computer isn’t working properly, then you can’t make the bootable Catalina USB installer. For this reason, you’ll need assistance from someone with a newer working Apple computer to perform the steps below. Please contact us if you’d like assistance.

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How to create a bootable installer for macOS

You can use an external drive or secondary volume as a startup disk from which to install the Mac operating system.

These advanced steps are primarily for system administrators and others who are familiar with the command line. You don’t need a bootable installer to upgrade macOS or reinstall macOS, but it can be useful when you want to install on multiple computers without downloading the installer each time.

Download macOS

Find the appropriate download link in the upgrade instructions for each macOS version:

macOS Catalina, macOS Mojave, or macOS High Sierra

Installers for each of these macOS versions download directly to your Applications folder as an app named Install macOS Catalina, Install macOS Mojave, or Install macOS High Sierra. If the installer opens after downloading, quit it without continuing installation. Important: To get the correct installer, download from a Mac that is using macOS Sierra 10.12.5 or later, or El Capitan 10.11.6. Enterprise administrators, please download from Apple, not a locally hosted software-update server. 

OS X El Capitan El Capitan downloads as a disk image. On a Mac that is compatible with El Capitan, open the disk image and run the installer within, named InstallMacOSX.pkg. It installs an app named Install OS X El Capitan into your Applications folder. You will create the bootable installer from this app, not from the disk image or .pkg installer.

Use the ‘createinstallmedia’ command in Terminal

NOTE: You’ll need to copy the full command provided below in step #3, and although it may seem to be on two or three separate lines it is all a single line of code.

1. Connect the USB flash drive or other volume that you’re using for the bootable installer. Make sure that it has at least 12GB of available storage and is formatted as Mac OS Extended.
2. Open Terminal, which is in the Utilities folder of your Applications folder.
3. Type or paste one of the following commands in Terminal. These assume that the installer is still in your Applications folder, and MyVolume is the name of the USB flash drive or other volume you’re using. If it has a different name, replace MyVolume in these commands with the name of your volume.
   Catalina:*
   sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia –volume /Volumes/MyVolume
   Mojave:*sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia –volume /Volumes/MyVolume
   High Sierra:*
   sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia –volume /Volumes/MyVolume
   El Capitan:
   sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia –volume /Volumes/MyVolume –applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app
4. Press Return after typing the command.
5. When prompted, type your administrator password and press Return again. Terminal doesn’t show any characters as you type your password.
6. When prompted, type Y to confirm that you want to erase the volume, then press Return. Terminal shows the progress as the bootable installer is created.  
7. When Terminal says that it’s done, the volume will have the same name as the installer you downloaded, such as Install macOS Catalina. You can now quit Terminal and eject the volume.

* If your Mac is using macOS Sierra or earlier, include the --applicationpath argument, similar to the way this argument is used in the command for El Capitan.

Use the bootable installer

After creating the bootable installer, follow these steps to use it:

1. Plug the bootable installer into a compatible Mac.
2. Use Startup Manager or Startup Disk preferences to select the bootable installer as the startup disk, then start up from it. Your Mac will start up to macOS Recovery. 
   Learn about selecting a startup disk, including what to do if your Mac doesn’t start up from it.
3. Choose your language, if prompted.
4. A bootable installer doesn’t download macOS from the Internet, but it does require the Internet to get information specific to your Mac model, such as firmware updates. If you need to connect to a Wi-Fi network, use the Wi-Fi menu  in the menu bar. 
5. Select Install macOS (or Install OS X) from the Utilities window, then click Continue and follow the onscreen instructions.

Learn more

For more information about the createinstallmedia command and the arguments that you can use with it, make sure that the macOS installer is in your Applications folder, then enter this path in Terminal:

Catalina:

/Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia

Mojave:

/Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia

High Sierra:

/Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia

El Capitan:

/Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia

Published Date (on Apple.com): March 12, 2020