News from loopy :
On read errors – I’ve found that a series resistor (10k or so) on the READ DATA pin (“pin 9” in my pinout diagrams) helps.

I built a GUI, it’s not quite finished but you can try it out. http://3dscapture.com/fdsstick/FDSStick.exe
I made a small change to the firmware to make using the button easier. Instead of slot numbers, you push the game number (1, 2, 3, etc..). After the game loads, the button changes disk sides (1=A, 2=B, …)