X failed when booting off USB drive (F7)

[tbl]

Hi there :)
I am currently trying to use F7 installed on External USB drive. It works well, but since i try to boot off my F7 from another computer, video detection failed ; then I need root passwd and new detection and so on... this is not a big deal, but definitly not good for a non-expert computing boss :-p

Is there something to do, to enable a kind of auto video detection ?

I need to make it work for my boss. If it works, he doesn't need to take his notebook but only this external usb drive, and will be able to boot off from any PC.

Any idea please ?
THX

[tbl]

The problem exactly comes from switching between i810 and Radeon !
If i set xorg to vesa, then autodetection works and change driver to i810 or to radeon, depending on the computer i use.