Q: How do I know if my pictures on 'photos' transferred to my external hard drive?

Launch Photos with the Option key held down, click on Other Library, and choose it. If everything's working properly and the library is being backed up to a second external drive, the photos can be deleted from the computer.

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If you double click on a library an edit opens then you are safe - but when you trash the one on the internal drive test one more time before emptying the trash

And if the iphoto library you migrated is on the internal drive you will not save much space at all as they are both using the same masters and previews - Photos saves disk space by sharing images with your iPhoto or Aperture libraries - Apple Support  --  so if you have an iphoto library on the internal drive you will have to move it too

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