It doesn't need to be formatted if writing the image to it as that contains all the partition information.

...fresh ROM for AstronBelt has it working better. Enemy craft shooting again, collision detect working. Only bad thing is that when the "disc" is changing scene I often get a flash of an explosion out of context for what's happening in the game. I tried deleting the DAT file and re-parsing the files but no joy. If anyone has any experience of this and fixing it, I'd appreciate any pointers.

Strangely Galaxy Ranger seems spot on.