Step 2: pull more stuff apart
In this picture, you can see the plastic case of the harmonica with the plates removed - place them to one side.
As the second picture so clearly illustrates: I'm an idiot. That flash drive isn't going to sit inside the harmonica without me cutting into one of the little valve thingies.
(NOTE: on the first try, I was able to shorten the 512mb flash drive by trimming 5mm or so off with the tree-trimmer thingies, and grinding one edge. This one has a funny black thing that covers the entire back, so I don't think I can cut through it without consequences.)
Oh well, find something smaller.
I've chosen the 128mb Tiny Pretec flash drive that happened to be on the desk, I also considered a bluetooth adaptor (perhaps one in each end!?) before realising it wasn't mine and putting it back together.