Boot from external HD

[elmo]

This is what I have in /root/grub/menu.lst to boot my ubuntu 
installation.   It works perfectly.
--------------------------------------------------------------------
# Ubuntu installation on /dev/sda2.
title        Ubuntu 7.10, kernel 2.6.22-14-generic (hda2)
root        (hd0,1)
kernel        /boot/vmlinuz-2.6.22-14-generic 
root=UUID=20f5f8e2-734e-43f4-8cbd-9d9c41ede634 ro quiet splash
initrd        /boot/initrd.img-2.6.22-14-generic
savedefault
boot
----------------------------------------------------------------
I also have a cloned copy of this same ubuntu on a USB external hard 
drive. 
Initially it was done to be a backup and possibly for copying to other 
computers
but now I would like to be able to boot it and run it from the external 
drive.

Since the external drive has a cloned copy, it seems to me that all that 
has to
be changed is the 'root' entry from (hd0,1) to whatever .is needed to 
designate
the partition on the external hard drive that has the cloned copy...In 
this case,
the clone ubuntu is on the second partition of the external drive.

My understanding is that external USB *__*HDs are SCSI so the 'root'
entry must somehow indicate that the ubuntu is on the second partition
on the external HD.

What should I use for the 'root' entry to accomplish this?
I should also mention that I have two hard drives installed in the
computer so the external drive is like a third hard drive.