"To my knowledge," the senior support tech at Apple said in answering 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 generally regard Apple support as being the best in the business. But even titans can fall.
My exchange with the tech had been prompted by a goal I had in mind: Installing 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 (a terabyte drive costs about $50) weighs ounces; my MacBook Air, with a case, charger and other accessories, weighs more than six pounds. I try to pack light.
Most of the material that showed up on a Google search dealt with older versions of OSX, the Macintosh operating system. I soon realized (wrongly, it turned out) that I would have to format my hard drive, which was running Sierra, the latest version, so I could install an earlier version. A word to the wary, and you should be wary: You can't install an earlier version of the operating system than the one your Mac came with. So, if your Mac shipped with Sierra, you can't revert to Mavericks. Why that is, I will leave to Apple programmers. Some day, I will climb Mount Cupertino and ask them other questions, too, like why they keep making subtle changes in Disk Utility that tend to flummox amateurs like me.
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. David A. Cox's YouTube tutorial has voice-over and screen shots for doing exactly what I wanted to do. I followed a 14-minute narration for doing some of the most arcane moves I've ever done on a PC. The moves in disco dancing pale in comparison.
The preparation part described in the tutorial took about half an hour and, after fiddling with more settings, I successfully booted into the USB drive. I was home free!
Not so fast.
I was able to launch the Yosemite installer, but the process hung. Hours went by, and still the installer bar didn't move, all the while telling me that the installation would complete in "about 17 minutes." It was stuck.
It never occurred to me that it the USB drive might be defective.
I tried instructions from another source that deviated slightly from the first. No luck. I went to the Apple support site and encountered code that brought me near to tears.
So much time wasted, so little to show for it.
And then I went to www.macworld.com, where I discovered three ways of doing what I wanted to do, one of which was for OSX Sierra _ my original system software. There, I found one line of arcane code, which I copied. I opened Terminal, pasted the code at the command prompt and in 30 minutes I was finished.
I booted from the USB drive (holding down the Option key as the MacBook booted). It booted just fine. But installing and then using the USB drive required far more patience that I had at that point. I was able to get to the desktop screen, but doing anything else ran so slowly as to make the experience useless. Just to make certain that the problem wasn't the USB drive, I tried the install on a USB 3.0 external hard drive. Something amazing happened.
Installing OS X on the external hard drive ran as quickly as it would have on the MacBook's internal solid state drive. Safari came up super-fast. Websites flashed on the screen. Dare I say it after all the frustration? "Yes!" I shouted, doing a victory lap around the computer room. "Success!"
So, by doing something I should have done in the first place _ try the installation on an external drive, rather than a thumb drive _ I was able to prove the Apple senior tech wrong.
Sir, if you are reading this, know that it's possible to make a boot and installation disk and run the Mac OS from a portable USB drive. I swapped out thumb drives, and the OS worked fine from that drive, too. It's also possible to use the either drive to install Sierra on any number of Macs. (IT folks, bless their pocket protectors, surely already know this.).
So now I take a portable hard drive or thumb drive when I hit the road, plug it into any Mac that has a USB 3.0 port and compute in complete privacy. I have a password for the drives, of course. And yes, I make a Time Machine backup of the portable drives when I'm back home.
For the code you need to use and other instructions for making a bootable thumb drive and installing Sierra on a portable USB drive, go to www.macworld.com/article/3092900/. I like the second choice, in which there's only one long command that makes the USB drive bootable and installs the OS.
