I burnt vastly too much time on this today, after having already figured this out a week ago, so here it goes in writing. In one sentence: format everything in exFAT and use the Master Boot Record partition scheme.
Spoiler for my Y3Q1 quarterly reflection: this year my dorm room is awesome. One of the new goodies that we have is a TV, and so I brought my basically unused Switch out of retirement to see if console gaming might be entertaining now. It is! We’ve been playing the hits (Mario Kart, Overcooked, etc.) and some indies. And as the Switch’s popularity among my friends has grown, so has the demand for more games.
This, of course, presents an issue: games take up much space, and the onboard storage of the Switch is not infinite. So I ponied up for an upgrade to my existing external storage (8GB) and bought a 128GB microSD card off Amazon. Of course, I had at this point forgotten how I set up the first card.
I took a big, painful detour trying to create a disk image of the old card with Disk Utility, SuperDuper, CCC7, hdiutil, and dd before realizing that I could just copy the data like any other files (Nintendo conveniently puts everything in one top-level folder).
After that, the partition scheme was just trial and error.
I’m looking forward to the expansion: my first round of purchases includes Stray, Factorio, and A Short Hike. If the habit proves more permanent, I’m sure my now-dormant Backloggd will make its way onto my homepage.