A 16GB or larger USB flash drive or a 16GB or larger partition on some other kind of external drive.
We've created a USB stick from both Mojave and Catalina, but your experience with other versions may vary. Luckily, it's not hard to make one-either with a handy graphical user interface or some light Terminal use. Further Reading macOS 10.15 Catalina: The Ars Technica reviewĪpple 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.
