The obvious question first - Are you sure the drive is not write-protected. There should be a small switch on it somewhere, make sure that it is not in the lock position.
But other that that, you fstab line looks OK. Have you tried as root?
My fstab line is this (My USB drive works correctly):
/dev/sda1 /mnt/usb vfat rw,suid,dev,noexec,noauto,user,async 1 2
I hope this helps