WBFS = official WKF firmware
FAT/FAT32 = swiss firmware
Swiss can only read SD cards formated to FAT/FAT32 and not compatible with WBFS format.
Swiss r253 or newer uses slot B as the default SD Gecko location unless you change the setting using "advanced" viewtopic.php?p=21442#p21442
does the above information help to resolve your issue?