If you switch to a library other than the System Photo Library, the changes you make are not reflected in iCloud Photos, because iCloud Photos only syncs changes from your System Photo Library. If you create any other libraries, make sure they are backed up using Time Machine.
WARNING: If a photo library is located on an external storage device, don’t use Time Machine to store a backup on that device. The permissions for the library may conflict with those for the Time Machine backup.
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