How to downgrade your Mac to an older version of OS X using an external drive

Installing an older version of OS X isn't as easy as you think it would be.
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Glenn Fleishman’s most recent books include Take Control of Zoom, Second Edition, Take Control of Your M-Series Mac, and Take Control of Securing Your Mac. In his spare time, he makes Tiny Type Museums. He’s a senior contributor to Macworld, where he writes Mac 911.