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:
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
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:
