I just tried dragging direct from my phone to DR - it won’t accept it either.
I think this relates to different connection protocols - and if you’re dragging direct from the Air, that will probably be the reason.
When you drag to DR, it doesn’t copy the whole file in any case. It creates its own “thumbnail” version, but needs to constantly refer to the original file for certain actions, and certainly for rendering. DR is rather efficient at working in this way, I find.
So if you were dragging from the air and it did accept it, you’d have to keep the Air connected and switched on at all times whilst editing. This obviously wouldn’t be very practical.
I have to admit I’ve never connected my MP to my laptop to copy files - I always remove the MicroSD and use that in my laptop’s card reader slot. It copies quicker, for one thing, and doesn’t need the MP switched on (the batteries are probably recharging after a flight in any case).
Let me know if copying to the computer first solves the issue.