Will do. I don't understand the "invalid header" error either. I messed with the rom with a couple of utilities but I'm not knowledgeable enough to understand the code. I can say that the MegaSD (with updated firmware) has a very good reputation for compatibility. I have about 465 games on there, not counting Sega CD and I rarely have an issue. Otherwise I wouldn't be bugging you. :) That said, I really appreciate you taking one more look at it. And congratulation on winning first place in the jam.
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Oh you don't need to buy it! Your help has paid for it many times over.
Firstly, for the other ones, i wasn't setting up he SRAM info correctly in the header. Which is odd, cos I had it working fine in a previous game, so I assumed I hadn't changed anything.. but somewhere along the line I'd deleted it!
But for all the others, I remembered vaguely that you have to pad the game out to the cartridge size, so I wrote a little tool to do that and patch up the header. I also fix the checksum in there, but I don't think that's used by anything. At least Kega Fusion doesn't complain about an incorrect checksum now though! :)