Hiya. So I finally got around to having a go at doing the the solderless board thing for this, and I'm pretty happy with how it turned out.
Was lucky enough to get a ribbon cable and connector that I could use to create the adapter like that, so it's super easy to route the cable out of the 3DS without need to create holes anywhere or try and wrangle something with the lanyard holes on the corners.
I went over the whole thing with the multimeter, testing from the pogo pins to the pins on the SD adapter to make sure that each path only led where it was supposed to and there were no crossings with any of them, and that's all fine. But I can't seem to be able to do anything with it now. The 3DS turns on properly when nothing is connected, and I can get it to trigger a 200 or 400 error by bridging the CMD and CLK/DAT0 contacts on the SD adapter with a pair of tweezers. But I can't seem to do get it to get the right error with the SD reader so that I can dump to the PC.
I was planning on using this Belkin SD card reader that we have, but it doesn't trigger anything on the 3DS. Occasionally I have gotten a 200 error (and once even an F/F/0/0 one :/) but no idea why it shows up sometimes and not most. Tried various combinations of plugging it into the computer before plugging it into the 3DS, trying to safely remove it, etc, but all I can manage is to trigger a 400 error but be unable to read it.
The other one I have is a Chronos brand multi card reader, and it usually triggers a 200 error when switching the 3DS on while plugged into it. Although now it seems to be opting for 200 instead. But it seems to take a ridiculously long time to settle after plugging into the PC, although it's perfectly fine when plugging it in with a normal SD card. Same goes for the other, actually.
So yeah. Any ideas on what else I could test to make sure there isn't something else wrong/any ways to fix it?