Download the macOS Sierra Installer
Launch the App Store app, then look for macOS Sierra in the store. Click on the Download button, and your Mac will download the installer to your Applications folder. If it automatically launches after download, quit the installer.
If you’ve already upgraded your Mac to Sierra, the installer is removed from the Applications folder. You can download it again if you go to Purchased in the App Store. Look for macOS Sierra in the list of apps that you’ve bought, and click on the Download button. If it automatically launches after download, quit the installer.
Use the Terminal to Create a Boot Disk
Connect the external drive to your Mac. I use Untitled to represent your external drive. If your drive is named something else, you need to change Untitled to the name of your drive.
sudo /Applications/Install\ macOS\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/Untitled --applicationpath /Applications/Install\macOS\ Sierra.app
- Terminal will ask for a password. This is your user password.
- Terminal will tell you that it will erase your drive. To confirm that you want to continue, type Y and hit Return.
- You’ll see that Terminal erases your drive. When that part is done, your Mac may ask you if you want to use the drive for Time Machine. Click Don’t Use.
- Terminal will copy the installer file to your drive. This will take a few minutes.
- After copying, Terminal is done. You should see Terminal display a “Copy complete” and Done notice. You can quit Terminal and your drive is ready for use.
How to boot from the installer drive
Plug your external drive into your Mac. Power up (or restart) your Mac. Press down on the Option key while the Mac boots. After a few moments, your Mac should display the Startup Manager, which will show you the available boot drives. Click on the external drive and hit Return. (You don’t need to select a network to proceed.)
Your Mac will display an OS X Utilites window. If you want to install Sierra and leave the data intact, select Install OS X. If you want to start over and wipe out the data, you need to go into Disk Utility to reformat the internal drive first, and then install macOS Sierra.