WonderSwan:WS Flash Masta USB

The FlashMasta is one of the oldest multirom flashcarts available for the WonderSwan. Instead of being microSD based, this cartridge has onboard storage and uses a microUSB cable to connect directly to your PC. It supports up to 16 slots for ROM files (The first game slot is usually reserved for the menu file so you can select more that one rom), and has one save file slot. Until recently, all games share the one save slot, meaning you lost progress when switching roms. But the new WSCartFriend menu automatically switches saves when changing games. This menu is experimental, but still recommended over the official menu.

Compatibility
Unfortunately, the FlashMasta does not have perfect compatibility with all WS ROMs. Below is a compatibility list pulled from the freeplaytech forums.

Due to hardware limitations, the following roms from GoodWSx v3.27 do not work with the WS Flash Masta v5.1: SRAM
 * Battle Spirit - Digimon Frontier (J) [!].wsc
 * Beat Mania (J) [M].ws
 * Cho Denki Card Battle Yofu Makai Kikuchi Shugo (J) [M][!].ws
 * Digimon Tamers - Battle Spirit (J) [!].wsc
 * Digimon Tamers - Battle Spirit V1.5 (J) [!].wsc
 * From TV Animation - One Piece - Grand Battle Swan Colloseum (J).wsc
 * From TV Animation - One Piece - Niji no Shima Densetsu (J) [!].wsc
 * Guilty Gear Petit (J).wsc
 * Makaimura (J) [M].ws
 * Meitantei Conan - Majutsushi no Chousenjou! (J) [M][!].ws
 * Meitantei Conan - Nishi no Meitantei Saidai no Kiki! (J) [M].ws
 * Meitantei Conan - Yugure no Koujo (J) [!].wsc
 * Rockman EXE N1 Battle (J).wsc
 * RUN=DIM Return to Earth (J) [!].wsc
 * WonderWitch [FreyaOS 1.2.1].ws
 * Yakusoku no Chi Riviera (J).wsc

EEPROM (save issues)
 * Anaza Hebun - Memory of Those Days (J) [!].wsc
 * Buffers Evolution (J) [M].ws >>patch below
 * D's Garage 21 Koubo Game - Tane wo Maku Tori (J) [M][!].ws >>use D's Garage 21 Koubo Game - Tane wo Maku Tori (J) [M][f1].ws
 * Digimon Digital Monsters for WonderSwanColor (J) [!].wsc >>patch below
 * Digimon Tamers - Brave Tamer (J) [!].wsc >>patch below
 * Digital Monster Card Game for WonderSwanColor (J).wsc
 * Digimon Tamers - Digimon Medley (J) [!].wsc >>patch below
 * Engacho! (J) [M][!].ws >> use Engacho! (J) [M][f1].ws
 * Fever Sankyo - Koushiki Pachinko Simulation (J) [M].ws
 * Final Lap 2000 (J) [M][!].ws >>use Final Lap 2000 (J) [M][f1].ws
 * Final Lap Special - GT & Formula Machine (J) [!].wsc >>use Final Lap Special - GT & Formula Machine (J) [f1].wsc
 * Flash - Koibito-kun (J) [!].wsc >>use Flash - Koibito-kun (J) [f1].wsc
 * From TV Animation - One Piece - Mezase Kaizoku Ou (J) [M].ws
 * Gensou Maden Saiyuuki Retribution - Hi no Ataru Basho de (J) [!].wsc
 * Golden Axe (J) [!].wsc
 * Gorakuoh TANGO! (J) [M].ws
 * GunPey (J) [M].ws >>use GunPey (J) [M][f1].ws
 * GunPey Ex (J) [!].wsc >>use GunPey Ex (J) [f1].wsc
 * Hunter X Hunter - Ichi O Tsugu Mono (J) [M][!].ws
 * Hunter X Hunter - Michibi Kareshi Mono (J).wsc >>use Hunter X Hunter - Michibi Kareshi Mono (J) [f1].wsc
 * Hunter X Hunter - Sorezore no Ketsui (J) [!].wsc >>use Hunter X Hunter - Sorezore no Ketsui (J) [f1].wsc
 * Inu Yasha Kagome no Yumenikki (J) [!].wsc
 * Itou Jun Ni Uzumaki Noroi Simulation (J) [M][!].ws >>patch below
 * Kinnikuman Nisei Choujin Seisenshi (J).wsc
 * Kinniku Man Second Generations Dream Tag Match (J) [!].wsc
 * Kurupara! (J) [!].wsc >>patch below
 * Last Alive (J).wsc
 * Lode Runner (J) [M].ws
 * Mikeneko Holme's Ghost Panic (J) [!].wsc
 * Nazo Oh Pocket (J) [M].ws
 * Nice On (J) [M].ws
 * Ouchan no Oekaki Logic (J) [M].ws
 * Pocket no Naka no Doraemon (J).wsc
 * Puzzle Bobble (J) [M].ws
 * Raku Jongg (J) [!].wsc
 * Ring Infinity (J) [M][!].ws >>patch below
 * Sennou Millenium (J) [M][!].ws
 * Shin Nihon Pro Wrestling Toukon Retsuden (J) [M].ws
 * Slither Link (J) [M][!].ws
 * Tare Pan no Gunpei (J) [M][!].ws >>use Tare Pan no Gunpei (J) [M][f1].ws
 * Terrors 2 (J) [!].wsc
 * Tetris (J).wsc >> Seemingly works when SRAM patched
 * Tetsujin 28 Gou (J) [M].ws
 * Tonpusou (J) [!].wsc
 * Turntablist - DJ Battle (J) [M][!].ws >>Turntablist - DJ Battle (J) [M][f1].ws
 * Uzumaki - Denshi Kaiki Hen (J) [M][!].ws
 * With You Mitsumete Itai (J) [!].wsc
 * X - Card of Fate (J) [!].wsc >>patch below

Patches can be found here.