I finally got my Wonderswan FlashMasta too and had the chance to actually perform a game flash attempt, using linux. While the FlashMasta is not plugged in to USB, the Flash Masta Desktop program performs normally. As soon as you plug it into a USB port, you get a "USB Acces" error. The solution is a bit silly and shouldn't be necessary but is easy. Just start the Flash Masta Destop program as root. Go to where the program is, open a terminal there and enter "sudo ./FlashMasta". The Flash Masta Destop program will start normally and show you your Flash Masta with all its slots.
P.S:: Whoops, it even says so in the source code readme, but nowhere inside the precompiled .tar.gz file
P.S:: Whoops, it even says so in the source code readme, but nowhere inside the precompiled .tar.gz file