I had this problem as well... still not sure if I've solved it or not. In my case, the best that I can tell is that the builtin maemo file manager breaks my cards when I use it to format memory cards. The 2 cards I'm using now finally allowed me to fill them up after I formatted them via a windows pc. I think that windows formatted them in FAT (works) while maemo file manager used FAT32 (corrupts them) ... no one responed to my original post about this so I guess I'm the only one with such problems.

However, I still have a third card that will only alllow me to write a certain amout to ... oddly, if i try to add one more average size mp3, i get an out of space error. However, when I tried copying a large podcast to the card it worked. At this time I haven't tried/tested any more with that card as I don't want to risk loosing what's currently on it.