How do I move Photos to external drive?

Caution: if you previously used any iCloud services, particularly iCloud Photo Library, you may not want to do this before taking other steps to avoid problems. If that is the case, please post those details before doing anything.

1. For best results, first make sure the external HD is formatted as Mac OS X Extended (Journaled).

2. Locate your Photos Library (usually in your Pictures folder) & copy it to the external drive. You may want to rename the copy afterwards to make it easier to find in the next step. "Photos Library 1" or anything that is enough different from the original to make it easy to identify will do.

3. Launch Photos with the option key held down & choose this Photos Library. You may need to use the "Open Other" button if it doesn't show up in the default list.

4. Test this library to make sure it behaves normally & has all the expected photos & videos in it. If that goes well, open Photos preferences & in the General section, click the "Use as System Photo Library" button. If you want to begin using any iCloud services, check the settings in the iCloud section.

5. I strongly suggest not deleting the original Photo Library until you are satisfied that the new one works as expected in all respects.

6. More cautions: be aware that you should backup the external library to another disk for safety's sake, & that your external drive should always be mounted whenever you open Photos.