iCloud & iCloud Photos · iCloud Photos and the Photos library
How do I copy my original photos out of Photos onto an external drive?
Short answer: Select photos and choose File → Export → Export Unmodified Original, then pick a folder on the external drive. With Optimize Mac Storage on, Photos downloads originals as it exports, so export in batches if space is tight.
Select the photos in Photos, then choose File → Export → Export Unmodified Original and pick a folder on the external drive. You get the original files exactly as they were imported, outside the Photos library.
With Optimize Mac Storage on, Photos downloads each original from iCloud as it exports, so it needs a connection and some temporary space — export in batches if your Mac is nearly full.
Exported photos on an external drive are a separate copy you control, which is useful as a backup or an archive. Storage Studio's Duplicates window can later spot exact copies among exported files outside the library.