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.
On a Mac with Apple silicon, starting up from an external storage device requires a working built-in startup disk. If your built-in startup disk is experiencing a hardware failure, you can’t use an external startup disk instead.
If your built-in startup disk has a different major version of macOS than your external startup disk, be aware that a more recent macOS version on one disk can install changes designed to keep your Mac secure, and these changes can affect your computer's ability to use a significantly earlier version of macOS on the other disk.
Make sure that your storage device is plugged into the appropriate port on your Mac.
If you're using a Mac with Apple silicon, plug your storage device into any compatible port except the DFU port. Learn how to identify the DFU port. After macOS installation is complete, you can connect your storage device to any compatible port, including the DFU port.
If you’re using any other Mac, plug your storage device into any compatible port.
A Mac startup disk requires either of these file system formats:
APFS format is recommended, especially if your storage device is a solid-state drive (SSD).
Mac OS Extended (Journaled) format, also known as HFS Plus, is also acceptable for traditional hard drives, and it's required for macOS Sierra 10.12 or earlier.
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.
This macOS installation will be specific to your Mac model. You should not expect it to start up other Mac models.
Make sure that the storage device is plugged into the appropriate port on your Mac.
Download macOS. If you can't do that because your Mac can't start up from its built-in startup disk, you can instead install macOS from macOS Recovery or from another Mac of the same model.
When the installer asks you to choose where to install, select the external storage device before continuing. You might need to click Show All Disks first.
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.
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.
When macOS software updates become available:
Make sure that your external startup disk is plugged into the appropriate port on your Mac.
Start up from your external startup disk instead of your built-in startup disk.
Use Software Update to update macOS on your external storage device.
Background Security Improvements are available only when your Mac is started up from its built-in storage. When started up from an external storage device, it receives these improvements as part of a subsequent software update.