Check ownership and permissions (use ls -l). Are you trying to write to it as user zaurus? Does the entries in the long listing look like this?
$ ls -l /mnt/card/
drwxr-xr-x 4 zaurus qpe 1024 Mar 27 03:15 Documents
or like this?
$ ls -l /mnt/card/
drwxr-xr-x 4 root root 1024 Mar 27 03:15 Documents
If they look more like the latter, then you will need to change its ownership by doing something like this as root (while the card is mounted):
# chown -R zaurus:qpe /mnt/card/
The -R makes the ownership change recursive through the subdirectories. You may not really want to make the whole card owned by zaurus, so instead you can pick and choose directories on the card:
# chown -R zaurus:qpe /mnt/card/Documents
# chown -R zaurus:qpe /mnt/card/home/zaurus
You can make a directory world-writeable without changing ownership by using chmod:
# chmod 777 /mnt/card/Documents