Just tested the Classic NES Castlevania and got the gamepak error, I have sent an email to Danny of the M3 team to see if they can fix it. Are there any games other than the classic NES that get this message on an emulator? as the M3 has a NES emulator built in so you can just play the actual NES games.
super mario advance 4 has another issue stating save data is corrupted and I dont even have a save for it yet... Not really a fan of imperfect emulation either.
Just tried Super Mario Advance 4, it did come up with the save error but once past it i could play the game and save fine, then when i reloaded the game it worked fine and the error did nto appear.