I'm not sure if you did this, but you need to add a line to your /etc/fstab that mounts the fat drive. You will need to make a folder you want to mount it to, usually /mnt/fat and that folder must have rw permissions for the user you want to have write access. You will also need to find out the name of your partition /dev/hda[num] or /dev/hdb[num], it depends on how many partitions/disk drives you have.
line in fstab that mounts fat w/ rw for users:
/dev/hdb1 /mnt/fat vfat users,defaults,umask=000 0 0
make sure you use the correct partition name.