How to Create an Emergency Mac OS Boot Device Using a USB Flash Drive

Use this backup tool to reinstall your operating system

A bootable copy of OS X or macOS on a USB flash drive is a great emergency backup tool to have on hand. If anything happens to your existing startup drive, your bootable installer will get you back up and running in no time.

A flash drive has several benefits over a bootable external or internal hard drive. First, a bootable flash drive works for both notebooks and desktop Macs. Flash drives are inexpensive, easy to use, and portable. With enough storage, a flash drive can even hold more than one version of OS X or macOS, so you can prepare an emergency installer for more than one Mac.

Here's a look at how to create a bootable flash drive installer as an emergency startup disk.

The information in this article covers creating emergency bootup USB drives for macOS CatalinamacOS MojavemacOS High Sierra, and OS X El Capitan.

Choosing and Formatting Your USB Flash Drive

Apple recommends using at least a 12 GB flash drive as a bootable installer, but a 16 GB flash drive might be worth the extra money. A 16 GB flash drive is big enough to install a complete copy of the macOS along with recovery utilities, such as Data Rescue, Drive Genius, and TechTool Pro, that you'd find helpful in an emergency bootup situation. If your budget allows, a flash drive larger than 16 GB certainly won't hurt.

Make sure whatever USB drive you select is formatted as Mac OS Extended. If it's not already in the right format, here's how to format your USB flash drive:

All data on your flash drive will be erased.

  1. With your USB drive plugged in, start up your Mac from macOS Recovery.

    Restart your Mac and immediately press and hold Command + R. When you see a startup screen, such as an Apple logo or spinning globe, release the keys. Enter a password if prompted. When you see the Utilities window, startup is complete.

  2. Select Disk Utilities from the Utilities window, then select Continue.

  3. From the list of drives attached to your Mac, select your USB flash drive device.

  4. Select the Partition tab.

  5. Select 1 Partition from the Volume Scheme dropdown menu.

  6. Type in a name for your flash drive.

  7. From the Format dropdown menu, select Mac OS X Extended (Journaled).

    Formatting options available in Disk Utility
  8. Select the Options button.

  9. Select GUID Partition Table from the list of available partition schemes.

  10. Select OK > Apply.

  11. A message will appear, warning that you are about to erase all data from the disk. Select Partition to continue.

  12. Disk Utility will format and partition your flash drive. When it's finished, select Quit. Your USB flash drive is now prepared to become a bootable OS X or macOS installer.

Download macOS

The next step is to download the operating system for which you want to make a backup and move it to your USB drive. The process differs slightly for different versions.

Catalina, Mojave, and High Sierra

  1. From the Mac App Store, download Catalina, Mojave, or High Sierra.

    downloading macOS Catalina
  2. Installers for each of these macOS versions download directly to your Applications folder. They'll be called Install macOS Catalina, Install macOS Mojave, or Install macOS High Sierra.

    The installer may try to open after it's been downloaded. If it does, quit it without continuing installation.

  3. Connect your USB flash drive to the Mac.

  4. Go to Applications > Utilities and open Terminal.

    Or, type Terminal into Spotlight Search to quickly open a Terminal window.

  5. In the Terminal window that opens, enter one of the following commands, depending on which macOS installer you're working with. Note that MyVolume is the name of your USB drive.

    For Catalina:

    sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

    For Mojave:

    sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

    For High Sierra:

    sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume

  6. After you enter the command, press Return.

  7. When prompted, type your administrator password and press Return again. Terminal doesn't show any characters as you type your password.

  8. 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.  

  9. When Terminal is finished, the volume will have the same name as the installer you downloaded, such as Install macOS Catalina.

  10. Quit Terminal and eject the volume.

El Capitan

When downloading El Capitan, the process is much the same. The only difference is that El Capitan downloads as a disk image. After you download El Capitan, open the disk image and run its installer, which is called InstallMacOSX.pkg. This process installs an app named Install OS X El Capitan into your Applications folder. Create your bootable installer from this app, not from the disk image, and follow the instructions as stated above.

Using Your Emergency Boot Device

To use the bootable flash device as an installer:

Make sure you're connected to the Internet during this process.

  1. Insert the USB flash drive into one of your Mac's USB ports.

  2. Use Startup Manager or Startup Disk preferences to select the bootable installer as your startup disk, then start up from it.

  3. Your Mac will start up to macOS Recovery. 

  4. If prompted, choose your language.

  5. Select Install macOS (or Install OS X) from the Utilities window.

  6. Select Continue and follow the onscreen instructions to install OS X or macOS on your Mac.

It's also possible to create bootable installers for earlier OS X versions, such as OS X Yosemite, OS X Mavericks, OS X Mountain Lion, and OS X Lion.