Add the following line to /etc/fstab:

/dev/sda1 /mnt/sda1 vfat defaults,users,noauto,umask=000 0 0

Then reboot (you don't actually have to reboot, but I forget the command, you might try "mount -a" instead of rebooting).

Now:

mkdir /mnt/sda1

Plug in your USB drive and type:

mount /dev/sda1 /mnt/sda1

and it should be writable. Hope what I said is correct.