A patch is now available to fix a couple of bugs found in Monster Hunter XX (Japan) for 3DS. Update patch 1.1.0 fixes a bug that occurs when crafting a certain weapon and alters the process for saving data. This is due to a particular issue that prevents the SD card from being recognized by the game and refusing to acknowledge any game save data. This udate is mandatory for anyone who wishes to continue playing online
Source: Perfectly Nintendo