How to create a bootable USB installer for macOS

Create a macOS installer USB drive for installing onto a new drive or booting a troublesome Mac from. This guide is for Macs running at least OS X Mavericks.

Since Apple released OS X Mavericks back in 2013 they thoughtfully added the terminal command createinstallmedia which can be used (with a downloaded macOS installer) to create an installer on a USB drive.

Firstly you'll need a USB drive of at least 8GB in size, 16GB is preferable. Bear in mind you'll be replacing the contents on your USB drive with the new macOS installer so make sure you've copied anything you want to keep off the USB drive first.

Next you need to download the macOS installer of choice using the following Mac App Store links:

• 11.x Big Sur

• 10.15 Catalina

• 10.14 Mojave

• 10.13 High Sierra

• 10.12 Sierra

Be patient, the installers are quite large so this will some time depending on the speed of your internet connection. The installer will be downloaded to your Applications folder.

Once downloaded, if the macOS installer starts, quit it. We just want the installer files, not to install it right now!

Next you need to connect your USB drive and format it:

• Connect your USB drive and make sure it mounts (and doesn't contain anything you need).

• Launch Disk Utility (press Command + spacebar and start to type Disk Utility).

• Note: If you are running High Sierra or later you will need to click on the View menu (on the top left in Disk Utility) and choose the Show All Devices option. Now you will see the USB root drive in addition to the USB data volume below it in the sidebar.

• Select your USB root drive in the sidebar (not the data volume). (And make sure you've selected your USB drive and not your main drive!)

• Click on Erase (like I said, make sure you are erasing your USB drive and not one of your other drives..).

• Choose Mac OS Extended (Journaled) as the Format.

• Choose GUID Partition Map as the Scheme.

• Name your drive MyVolume

• Click on Erase and wait for Disk Utility to format your USB drive.

• Click Done.

Now you're going to use the createinstallmedia command to create the installer.

• Open Terminal (press Command + spacebar and then start typing Terminal).

• Highlight and copy the command from below for your chosen version of macOS:

Big Sur

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

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

Sierra

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

• Paste the command into the Terminal window.

• Click Enter/Return.

• Terminal will ask you to enter your password. Note you won't see the characters appear as you type. Click Enter/Return when you've typed your password.

• Terminal will warn you that it's about to erase the drive. If you want to continue press Y and then Enter/Return. The process can take a while, you'll see Erasing Disk: 0%… 10%… 20%… 30%…100%… (you are, after all, writing a large installer to a USB drive so be patient!)

Congratulations, you've just created a macOS USB Installer.

Now you can boot a Mac (with the USB plugged in) by holding down the alt key to show the boot menu from where you can select the OSInstaller USB drive to boot from (and then install macOS).

More information:

https://support.apple.com/en-gb/HT201372