Can’t install Windows Server 2003 trail on a Laptop USB disk.

I am currently short of disk space on my Toshiba Laptop satellite A30 Series. I am studing for MCSE course, so I bought a mobile disk with 80G and connected to Laptop via USB port. I have managed to install Windows Server 2003 trail version on the mobile disk. However, I am unable to boot it from USB, a message appearing tells me something that it can not recognise the disk.
Note that the BIOS version v1.3.
Does any body have any idea?

Thanks