Thanks. After messing with it some more, I'm blaming it on a picky SD slot, which I can deal with; I won't pursue further unless I decide it's something more than that.
Otherwise, once it's happy with how I've inserted the SD, it's awesome, I have no complaints. I'm so happy to be free of all the concerns and caveats of the CD drive, beyond the usual benefits a flash cart offers, plus the Jag's unique open license status... Yeah, a quirk can be an endearing thing in the long run, anyway.