Apple hasn’t shipped operating systems on physical media in a full decade, but there are still good reasons to want a reliable old USB stick for macOS Catalina. Luckily, it’s not hard to make one—either with a handy graphical user interface or some light Terminal use.
Here’s what you need to get started:
If you want to use this USB installer with newer Macs as they are released, you’ll want to periodically re-download new Catalina installers and make new install drives. Apple rolls support for newer hardware into new macOS point releases as they come out, so this will help keep your install drive as universal and versatile as possible.
There’s also one new consideration for newer Macs with Apple’s T2 controller chip—as of this writing, that list includes the iMac Pro, the 2018 Mac Mini, the 2018 MacBook Air, and 2018 and 2019 MacBook Pros, though Apple keeps an updated list here. Among this chip’s many security features is one that disallows booting from external drives by default. To re-enable this feature, hold down Command-R while your Mac reboots to go into Recovery Mode and use the Startup Security Utility to “allow booting from external media.” If you’re trying to install an older version of macOS, you may also need to go from Full Security to Medium Security to enable booting, but if you’re just trying to install the current version of macOS, the Full Security option should be just fine. And if you’re just doing an upgrade install rather than a clean install, you can run the Catalina installer from the USB drive from within your current installation of macOS, no advanced tweaking required.
The easy way
Once you’ve obtained all of the necessary materials, connect the USB drive to your Mac and launch the Install Disk Creator. This app is basically just a GUI wrapper for the terminal command, so it should be possible to make install disks for versions of macOS going all the way back to Lion. In any case, it will work just fine for our purposes.
Install Disk Creator will automatically detect macOS installers on your drive and suggest one for you, displaying its icon along with its path. You can navigate to a different installer if you want, and you can also pick from all the storage devices and volumes currently connected to your Mac through the drop-down menu at the top of the window. Once you’re ready to go, click “Create Installer” and wait. A progress bar across the bottom of the app will tell you how far you have to go, and a pop-up notification will let you know when the process is done. This should only take a few minutes on a USB 3.0 flash drive in a modern Mac, though using USB 2.0 will slow things down.
The only slightly less-easy way
The Install Disk Creator is just a wrapper for the terminal command to create macOS install disks, so if you’re comfortable formatting your USB drive yourself and opening a Terminal window, it’s almost as easy to do it this way. Assuming that you have the macOS Catalina installer in your Applications folder and you have a Mac OS Extended (Journaled)-formatted USB drive (which is to say, HFS+ and APFS) named “Untitled” mounted on the system, you can create a Catalina install drive using the following command.
sudo /Applications/Install\ macOS\ Catalina.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled
The command will erase the disk and copy the install files over. Give it some time, and your volume will soon be loaded up with not just the macOS installer but also an external recovery partition that may come in handy if your hard drive dies and you’re away from an Internet connection. If you would like to create an install drive for a macOS version other than Catalina, just tweak the paths above to refer to Mojave or High Sierra instead.
Whichever method you use, you should be able to boot from your new USB drive either by changing the default Startup Disk in System Preferences or by holding down the Option key at boot and selecting the drive. Once booted, you’ll be able to install or upgrade Catalina as you normally would. You can also use Safari, Disk Utility, or Time Machine from the recovery partition to restore backups or troubleshoot.