Don't have a usb hd, but you need at least this line in fstab for usb support:
none /proc/bus/usb usbdevfs defaults 0 0

For my thumbdrive I have in fstab:
/dev/sda1 /mnt/sda1 vfat noauto,users,exec,umask=000,uid=markus,gid=markus 0 0

And as root do:
cd /mnt
mkdir sda1
chmod 777 sda1

I'm assuming here your thumbdrive is also sda1, if not, change to what you have.
To find out do after plugging it in:
cat /var/log/syslog |grep sda