That "No mixing saves" is bullshit, my 251 has 245 blocks filled with European saves and 6 blocks with US saves, another card of mine houses both European and Japanese saves. I reformatted the 59 for Japanese usage but no PAL game complained about making saves to it. I haven't tried US and Japanese saves on one card yet but I doubt that'll be much of an issue, either. For reference, my Cube is a PAL model.
If a game gives you trouble about reformatting, give it another card, reformat that and copy your save over to your main card (for convenience's sake)
It could be your memcard being defective, copying saves around doesn't corrupt them, no matter whether your card is formatted for western or eastern usage.