Use an external storage device as a Mac startup disk
If you have an external storage device, such as a USB or Thunderbolt drive, you can install macOS on that device, then use it as a Mac startup disk.
Connect your storage device to your Mac
Check the format of your storage device
Install macOS on your storage device
Allow booting from external media
Select your storage device as the startup disk
Keep macOS up to date on the storage device
Connect your storage device to your Mac
Make sure that your storage device is plugged into the appropriate port on your Mac.
Connect to a Mac with Apple silicon
If you're using a Mac with Apple silicon, your Mac has one or more USB or Thunderbolt ports that have a type USB-C connector. While you're installing macOS on your storage device, it matters which of these ports you use. After installation is complete, you can connect your storage device to any of them.
Mac laptop computer: Use any USB-C port except the leftmost USB-C port when facing the ports on the left side of the Mac.
iMac: Use any USB-C port except the rightmost USB-C port when facing the back of the Mac.
Mac mini: Use any USB-C port except the leftmost USB-C port when facing the back of the Mac.
Mac Studio: Use any USB-C port except the rightmost USB-C port when facing the back of the Mac.
Mac Pro with desktop enclosure: Use any USB-C port except the one on the top of the Mac that is farthest from the power button.
Mac Pro with rack enclosure: Use any USB-C port except the one on the front of the Mac that's closest to the power button.
Connect to any other Mac
Plug your storage device into any compatible port.
Check the format of your storage device
A Mac startup disk requires either of these file system formats. You can use Disk Utility to select your storage device and see which format it's using. You can also use Disk Utility to erase it using either format.
APFS format is recommended for macOS High Sierra or later, especially if your storage device is a solid-state drive (SSD).
Mac OS Extended (Journaled) format, also known as HFS+, is also acceptable for traditional hard drives, and it's required for macOS Sierra or earlier.
Install macOS on your storage device
This macOS installation will be specific to your Mac model. You should not expect it to start up other Mac models.
Make sure that your storage device is plugged into the appropriate port on your Mac.
Download macOS via the App Store or a web browser. If you can't do that because your Mac can't start up from its built-in startup disk, you can install macOS from macOS Recovery instead.
When the installer asks you to choose where to install, select your external storage device before continuing. You might need to click Show All Disks first.
Allow booting from external media
If you're using a Mac with the Apple T2 Security Chip, use Startup Security Utility to allow your Mac to start up (boot) from external or removable media. Learn how to use Startup Security Utility.
Select your storage device as the startup disk
After installing macOS on your storage device, you can select that storage device as your startup disk. Your Mac can then start up from it instead of its built-in startup disk.
Keep macOS up to date on the storage device
When macOS software updates or upgrades become available:
Make sure that your storage device is plugged into the appropriate port on your Mac.
Start up from your external storage device instead of your built-in startup disk.
Use Software Update to update macOS on your external storage device.
