How to move an Apple Photos for OS X library to an external drive

A reader would like to host his Photos Library on a drive other than the startup volume.
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    Related:
  • Photography
  • Apple Photos
  • Mac
Glenn Fleishman’s most recent books include Take Control of iOS & iPadOS Privacy and Security, Second Edition (covering iOS 15/iPadOS 15), 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.