I'm having the same problem with the Micro SD cards for my Supercard Lite. It happened with different Micro SD cards (one 1 GB card and one 2 GB card), so I know it's not the card. And it happens when I try to fill the cards with ROMs off different computers and using different card readers, so I know it's neither of those, either.
I just formatted the card, as suggested, but when I try to add files, it stops after 449 MB. I'm trying to add 560 MB of files in total to the empty card. Right now, the card shows 1.45 GB of free space.
Any ideas would be most appreciated. Thanks!
EDIT: I poked around on another board and found the answer:
It's to do with the FAT file system that the card needs to use. I just put the rest of the games in a sub folder and all works fine now.
Indeed, that's what I did. I made three folders and divided my ROMs between them. Then, instead of copying all the files directly to the card, I copied the three folders (with the files in them) over to the card - it worked just fine. I hope this works for you too, Richter.