Booting from a CD (ISO Image) on to USB external Hard drive ?????

[kumarullal]

Here is my question in detail

A) I attach a 4 port USB 2.0 Hub to Shevaaplug
B) Next, On one port of the HUB I attach an External CDOM drive (With the ISO Image CD, of an application built on Debian or FreeBSD or Ubuntu)
C) On the other port of the USB HUB I attach a 1 TB Hard Drive
Now my question is, can we configure the UBOOT settings such that, the first preference for booting is from the CD ROM Drive and Next prerefence is to boot from the external Hard Drive.
So That when Sheevaplug boots, it straight goes to the CDROM with the ISO Image cd.
WHen I select it to run, The image is copied to the external 1 TB hard drive and  NOT to the local drive of Sheeval plug.
How is this achieved?
Thanks in Advance.
Thanks