Canadian AT-100, file manager freezes

[Z0mb13]

When I try to copy files to a SD card, the file manager freezes. I can copy smaller files, (even a small video up to 6.29mb) but anything larger causes the file manager to freeze. Even when I hit the 'cancel' nothing happens. If I remove the SD card, it unfreezes and goes back to the internal memory. The SD card is formated fat32, with 64kb unit allocation size.
Why can't I copy a 20mb file to this SD card?

Thanks for your help.