Well, I dont' have an M3, but I'll give it a shot. Do you just have the M3? Do you have a Superkey? If not, then that is your problem. M3 Simply is a slot 2 device I beleive, and a Superkey is needed to boot files from slot 2 in DS mode. A little more information would be helpful in solving this. Sorry if I'm not any help, I have a GBAMP.
Bookmarks