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With Lightroom specifically? Not really. You could export things straight to the external drive and then delete them, if you wanted to keep the iPad storage clear. Or there’s an Adobe app you can use to download your Lightroom library to an external drive attached to a PC, if it’s the backup you’re after. I don’t know how it is with all the possible photo editing apps out there, but my general experience has been that you have to import things to the iPad app and work on them locally, but you can export them wherever you want after that.
I have defaulted to doing most of my editing on Lightroom CC on my iPad Pro because it’s a lot easier than doing it on a PC for ad hoc edits. Using a touch screen is much quicker and more intuitive for individual photos, and the performance/battery life is better than on my (aging) surface pro for this specific application.
It’s mostly great, although I sometimes do find that there’s functionality that is PC only. I think if I needed to do big batch edits, wanted to use keyboard shortcuts or an editing deck, or if I had a preference for one-and-done software purchases over app subscriptions I would just stick to a computer.