Choose and Format 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.
How to Format Your USB Flash Drive
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:
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. Select Disk Utility and then select Continue. From the list of drives attached to your Mac, select your USB flash drive and then select Erase. Type in a name for your flash drive. From the Format drop-down menu, select Mac OS X Extended (Journaled), and then select Erase. Disk Utility will format your flash drive. When it’s finished, select Done and quit Disk Utility. 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
From the Mac App Store, download Catalina, Mojave, or High Sierra. 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. Connect your USB flash drive to the Mac. Go to Applications > Utilities and open Terminal. Or, type Terminal into Spotlight Search to quickly open a Terminal window. 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 After you enter the command, press Return. When prompted, type your administrator password and press Return again. Terminal doesn’t show any characters as you type your password. When prompted, type Y to confirm that you want to erase the volume, then press Return. Terminal will ask for permission to access files on a removable volume. Select OK to continue. Terminal will show its progress as it transfers macOS to the USB device. When Terminal is finished, the volume will have the same name as the installer you downloaded, such as Install macOS Catalina. 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.
For Catalina:
For Mojave:
For High Sierra:
Use Your Emergency Boot Device
To use the bootable flash device as an installer:
Insert the USB flash drive into one of your Mac’s USB ports. Use Startup Manager or Startup Disk preferences to select the bootable installer as your startup disk, then start up from it. Your Mac will start up to macOS Recovery. If prompted, choose your language. Select Install macOS (or Install OS X) from the Utilities window. Select Continue and follow the onscreen instructions to install OS X or macOS on your Mac.
