Tinkering with an ASUS Transformer here, W8.1. I've run RB with a dozen audio tracks or so on it as described in the original post. Same USB3 port on docked keyboard type setup. With charger plugged in the external drive didn't seem to be much of a drain. <grin>
I think next I'll try installing RB to a 64 gig sd card and see how it runs from there.

The cursor lags a bit, but it works.
Recording to it would be a different question. I wouldn't want to battle that. Not recording audio anyway. It's a basic model; only 2 gig RAM, 32bit OS and Atom processor.

I'd bet the Surface 3 could record a couple audio tracks, and maybe the Switch could too. It'd be interesting to know.

I do not work here, but the benefits are still awesome
Make your sound your own!