Make sure that your storage device is properly formatted
For best results, your external hard drive, thumb drive, SDHC or SDXC card, or other storage device should be formatted as Mac OS Extended, not FAT, ExFAT, or NTFS. And to function as a startup disk, it needs to be using a GUID partition map. Here's how to check:
- Connect your external storage device to your Mac.
- Choose Apple menu () > About This Mac, then click the System Report button. Or hold down the Option key and choose Apple menu > System Information.
- Select Storage from the sidebar, then select your storage device from the list of volumes. Make sure that the Partition Map Type shown for your storage device is GPT (GUID Partition Table):
If your storage device isn't using a GUID partition map, you can follow these steps to create one with Disk Utility. This deletes all of the data stored on your device.
- If you're using OS X El Capitan or later, select your external storage device in Disk Utility. (Select the disk name, not the volume name indented beneath it.) Then click Erase and choose the GUID partition map scheme and any Mac OS Extended format before erasing.
- If you're using OS X Yosemite or earlier, select your external storage device in Disk Utility. (Select the disk name, not the volume name indented beneath it.) Click the Partition tab, select your partition, then click the Options button. Select GUID Partition Table before partitioning.
Install macOS on your storage device
Follow the steps for reinstalling macOS, but select your external storage device when the installer asks you to choose a destination disk. The installer needs at least 8GB of available storage space.
This installs a version of macOS that can start up your Mac. It can also start up another Mac, if it's the same macOS version that came with that Mac, or a compatible newer version.
Select your startup disk and start up from it
After installing the operating system on a properly formatted storage device, choose one of these methods to use that device as the startup disk for your Mac.
Use Startup Disk preferences
When you use Startup Disk preferences to select a startup disk, your Mac starts up from that disk until you choose a different one.
- Choose Apple menu > System Preferences, then click Startup Disk.
- Select your startup disk, then restart your Mac.
Use Startup Manager
When you use Startup Manager to select a startup disk, your Mac starts up from that disk once, then returns to using the disk selected in Startup Disk preferences.
- Hold down the Option key immediately after turning on or restarting your Mac.
- Release the Option key when you see the Startup Manager window.
- Select your startup disk, then click the arrow or press Return.
If you can't select your startup disk or start up from it
Some third-party external devices use Option ROM firmware. To enhance system security, Mac computers with up-to-date software don’t automatically load Option ROM firmware. As a result, your Mac doesn't see devices that have Option ROM firmware until you load the firmware by pressing Option-Shift-Command-Period at the Startup Manager window. Do this each time you want to start up from the device or from a startup disk connected to it.
If you're using an Early 2015 (or older) Mac model, you can disable this security feature and allow your Mac to load Option ROM firmware automatically. This removes an important protection against potential unauthorized access to your Mac by other people with physical access to it.
- Open the Terminal app, which is in the Utilities folder of your Applications folder.
sudo nvram enable-legacy-orom-behavior=1, then press Return.
To undo this command, enter
sudo nvram -d enable-legacy-orom-behavior.
- If your external storage device is slower than your built-in startup disk, your Mac might take longer to start up, and it might not perform as quickly.
- If you can't start up from your built-in startup disk, you might be able to start up from macOS Recovery, then use the utilities in macOS Recovery to set up a startup disk.
- To use an external USB drive as a startup disk, the drive must have Mac OS X Tiger v10.4.5 or later installed, and your Mac must have an Intel processor.
- If your Mac is using OS X Lion v10.7.3 or later, you can use Startup Manager to start up from your Time Machine backup disk. Startup Manager identifies your Time Machine backup as ”EFI Boot.”
- You can also create a bootable installer for macOS.