Wish I’d seen this 3 days ago. Needed to backup our Shared Library and did the following (about 10K photos/videos at 300gb, had enough space so full downloaded to MacBook, not optimize. 1. Repair iPhoto library, (Wait 24 hrs to re-sync to iCloud) - initially looks like it moved all photos to personal. 2. Select small chunks, by year worked well enough - selecting All gave me the spinning ball. Then export unmodified to external hdd into folders organized by year. 3. Moved entire photo library file to another external hdd. 4. Open iPhoto and select external hdd library as primary library, let it re-sync to iCloud (Wait 24 hrs). iPhoto now running off external HDD library and I’ll backup to separate external HDD monthly. Repair function and wired Ethernet connection were biggest game changers to previous attempts.