HI FF,
Grub does not read the file /etc/fstab, will read only the file /boot/grub/menu.lst, and will search the label there.
Once you boot from your external hard disk, the file that will be used will be the /etc/fstab from your external HD.
You can however retain the entry in your current /etc/fstab, only change the option from "default" to "noauto",
this will prevent the automatic mount at boot time.
AS