hi.. i think i have the similar problem
i try to make a data logger using microcontroller, the code is already tested using micro SD Card, and the result is very good, but when i try to used MMC Card, it doesn't work..
then i found that the initialisation is little bit different, and now i already init the MMC Card - result ok, but i still could not read or write to the MMC Card,,
to your experience, what's wrong in the code? because using SD Card, it's work..