The one thing iOS can do with external storage devices is import photos: if you plug in a camera or a memory card from a camera, iOS 12 will automatically pop open the camera import screen and let you import photos into your camera roll.
That’s it. That is the sole way iOS 12 can address external storage. And to make matters worse, you are required to import to the system camera roll — you can’t import photos directly into an app like Lightroom CC. Apple has to be in the middle.
I use Lightroom CC all the time and I would love to manage and edit all my photos on an iPad Pro, especially since editing with the Apple Pencil is so much fun on this display. But I have no desire to import hundreds of RAW files into my camera roll and iCloud photos account. When I brought this up, Apple very proudly pointed to a new Siri Shortcut from Adobe that imports photos from the camera roll into Lightroom and then automatically deletes them from the camera roll.
I couldn’t test that Lightroom Siri Shortcut, since it’s not yet available. But I can tell you that macro-based hacks around the limitations of an operating system are not usually included in bold visions of the future of computing, and that Siri Shortcut is a pure hack around the limitations Apple has imposed on the iPad Pro.
Oh, but it gets worse. I shoot photos in JPG+RAW, and the iOS PhotoKit API only allows apps to grab one or the other from the camera roll. So I could only import my RAW images into Lightroom, leaving the JPGs behind to clutter up my camera roll and iCloud storage. That’s untenable, so I just gave up and imported everything directly into Lightroom using my Mac, because my Mac doesn’t insist on abstracting the filesystem away into nonsense.