to make the save data go to a Flashdrive?
Saved data automatically go to a subdirectory of the user's directory (see FileTools::get_base_write_dir()).
But from your Lua scripts, you can use the regular Lua file API to manipulate savegame files. Open the savegame file with sol.file.open() (http://www.solarus-games.org/doc/latest/lua_api_file.html#lua_api_file_open) and then just copy its content wherever you want.