Booting a Mac from a thumb drive
By Harold Glicken
“To my knowledge,” the senior support tech at Apple said, in answer to my question, “you can’t boot into an Apple computer from a USB drive.”
“Whoa. The Internet is full of ways of doing just that,” I responded.
“Good luck, then.”
I had a goal in mind: Install Sierra, the latest Macintosh operating system , along with essential email and other apps, on a USB drive, so that I could leave my MacBook at home, and just bring the USB drive with me when I go abroad. Once I had done that, I could connect the USB drive into any Mac with a USB 3.0 port and boot from the portable drive. I wanted to be able to use the host Mac without leaving any traces of my computing. A portable external USB drive or thumb drive weigh ounces; my MacBook Air, with a case, charger and other accessories, weighs more than six pounds. I try to pack light.
It goes without saying, but I’ll say it anyway: Make a backup of your important files on a separate disk before you install any upgrade or downgrade.
One of the best tutorials for making a USB drive bootable is for Yosemite users only. I followed a 14-minute YouTube narration for doing some of the most arcane moves I’ve ever done on a PC. The moves in disco dancing pale in comparison.
There are no guarantees in computing. The following, gleaned from and several other sources, worked for me; I hope it works for you as well:
1. Format a minimum 32-gig external hard drive or thumb drive. Memory is cheap; buy as much as you can afford. Make sure you format the drive Mac Journaled and the GUID partition part is checked. Change the name of the drive to Untitled.
2. Check your Mac's Applications folder to see if you have the Sierra installer. If not, go to the app store and download Sierra -- but don't install it. Put the installer in your Applications folder.
3. Bring up Terminal (go to the search bar and type terminal to bring it up).
The following instructions are directly from MacWorld:
At the Terminal prompt, type:
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 doesn’t display characters when you type it in. Hit Return.
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.
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 Utilities window. To install Sierra and leave the data intact, select Install OS X.
By following MacWorld's instructions to the T, I am now running Sierra from a thumb drive.