[chrischen]

Photos app for offline storage is also woefully neglected by Apple. They don't really design it for use with an external drive (you are expected to locate your library on a internal storage or permanently attached storage). The reason is that the photo library constantly gets corrupted on external drives, forcing a long rebuild/repair process.

Often the photos app doesn't even detect the iphone even when plugged in, and it's a serious bug that Apple has neglected for years.

[geekifier]

Everything Apple does seems to be designed to drive hardware sales. Why support external drives, when you can be up-sold for larger internal storage (at a huge markup)? The "Photo Library" could simply be a database file with references to photo locations, alas that might confuse Mac/iOS user with "files" vs "photos".