I'd try a proper SD Formatting Tool application to ensure the SD card is
formatted to FAT32, and also make sure you've added "SD" in the list of
servers in the Fujinet. I personally don't use any SD and just do
everything via TNFS on the PC. It runs on Windows, Mac, Linux, RPi, you
name it. No need to remove/insert/copy/whatever. It's very convenient
for copying stuff out of ATRs and onto the "hard drive" on the Side B ..
err, excuse me, Side 3 cartridge.