No luck with saving game on cart for U64 so far. The only thing that works for me is saving to a d64 disk file. Not sure what the issue is. Maybe I need a physical EasyFlash3 Cart and load the game onto that for it to work? I think I got Sam's Journey Gmod2 cart to save, but that is a different format.
Viewing post in Briley Witch Chronicles (C64) comments
When the game saves to EasyFlash, the U64 only writes that into RAM, to save this across power cycles, you have to manually save it back to the SD in the U64 menu. So overall, saving to disk is probably more convenient on the U64. (This is a limitation of the U64 EasyFlash support and also applies to other EasyFlash titles when played on the U64.)