boot GParted-livecd off EXTERNAL-USB-CD

ATM I forgot to add this option in GRUB menu !
So if you need to boot off external usb cd, just edit the first line of GRUB, (press "e" twice), then find the string "cdroot" and change it to "cdroot=/dev/sr0"  (0=zero !!!) [ us keymap at this point !!).
Then press escap and "b" to boot.
It should do the trick