The weird thing about potential external drive compatibility is that you wouldn't be able to do the whole "switch" thing.
I could see Nintendo only allowing SD cards as additional storage for that reason.
Yeah I was thinking this too. Only issue would be that SD cards are still relatively expensive when you get into the 128GB/256GB range. I wonder if they could do something like being able to have several different SD cards for different digital games/patches. Like you buy 1 64GB SD card and it has your digital version of game x, y, z, and you have another SD card that has games a, b, and c. And then you could just swap them out, with each card being tied to the console. Probably not though, curious how they'll handle storage.
Edit: For clarification, I'm not saying these are the carts Nintendo sells. It would basically be the same as people storing their games on external HDDs, just with SD cards.