Latitude D630 - USB memory stick problem - 16Gb bootable?

Hi,

I have a Dell Latitude D630 that I want to boot an image from a USB memory stick, the stick is formatted with bootable option in winpe2008, but when booting up, selecting F12, it just hangs. A white underscore blinking (as in DOS) is all that is shown. To me it looks like the device needs power to operate fully. An 8Gb stick works flawlessly. Have used an external HDD with its own power, and it works great. Any ideas? Thanks.