cannot boot from usb

Hi list,

I use Ubuntu 10.04


I downloaded an usb image from http://rom-o-matic.net/ and wrote it to usb:

sudo dd if=gpxe-1.0.0-sis900.usb of=/dev/sdb1

I changed the bios to boot from the usb but it doesn't work. It goes  
to the next boot device (harddisk). I'm sure I changed the boot order  
in the bios correctly.

Thanks







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