Bootable clones of your boot drive should be part of any serious backup scheme. These drives are invaluable if your system suddenly fails to boot, giving you troubleshooting options and even permitting you to keep working on an identical copy of your working files. While bootable copies are best made on internal SSDs and HDDs, they can also be made from USB drives. Whether external HDDs or solid-state chip storage, you can use a compatible USB drive to install macOS.
Installing macOS on a USB Drive
1. Format your USB drive as APFS or JHFS+. Choose the GUID Partition Table or GPT as your partition table.
2. Download the version of macOS you want to install. Unless you have a specific reason to install an older version of macOS, you can download the most recent installer from Apple. Look for the installers labeled “Combo Update” which contain the full installer. These don’t require a pre-installed version of macOS to run.
3. Open the installer you downloaded.
4. Click “Show all disks” to reveal your USB drive if necessary.
5. Select your USB drive in the installer options.
6. Enter your administrator password and follow the steps to finish the installation on your USB drive.
7. When prompted, restart your Mac.
Your Mac should boot from your USB drive by default to continue the installation. If it doesn’t, hold down the Option key to enter the Startup Manager, then select the USB drive. This ensures the installer is able to proceed properly.
Cloning Your Hard Drive to a USB Drive
If you have a USB drive large enough to accommodate the data on your boot drive, you can create a direct clone of your boot drive. If you use the proper software, this disk will also be bootable. You can use either SuperDuper or Carbon Copy Cloner to create bootable clones of your boot drive. This example will use Carbon Copy Cloner (CCC) to create a clone of your hard drive.
1. Select your boot drive as the source for the clone.
2. Set your USB drive as the target for the cloning operation.
3. Click “Clone” to start the cloning process. The content of the USB drive will be overwritten and replaced with the clone of the boot drive.
Booting from a USB Drive
Once you’ve installed macOS on a USB drive or cloned your hard drive to a USB drive, you will need to restart your computer and boot from the clone drive to use it. You can run from a bootable USB installation of macOS just like running from an internal SSD: there’s no operational or logical difference between the two systems. But running off a USB drive is invariably slower than running off an internal SSD.
Set your bootable USB as your startup disk for the next boot in “Apple Menu -> System Preferences -> Startup Disk,” then click “Restart” to reboot your Mac. Your Mac will reboot from the USB instead of your default startup disk.
Alternatively, you can also use the Startup Manager to select your boot drive when you start macOS.
1. Restart your Mac and hold down the Option key to enter into the boot selection dialog.
2. Use your keyboard’s arrow keys to select the USB drive from the list of bootable devices.
3. Your USB drive will begin to boot.
Don’t be surprised by the slowness – it will take considerably longer to boot the operating system of a USB drive than the PCIe SSD installed in modern Macs.
You can now run the operating system as normal and can use this USB drive to boot up your Mac if your boot drive fails. You can also perform disk operations on the boot drive safely from a USB stick.