En lisant la doc de XXClone j'ai trouvé une réponse, Microsoft par design n'autorisait pas de démarrer sur un disque externe. Je vais tout de même essayer...


"XXCLONE makes the external (USB) disk self-bootable.


However, the disk must be directly attached to your computer in order to become the Windows system disk. That is, you need to take the bare disk drive inside the external enclosure, and attach it directly to your computer as an internal disk in order to have the disk succeed in booting your computer.
Apparently, Microsoft does not like to see a Windows booted from an external disk. We suppose this is a deliberate design to prevent software piracy. Under normal circumstances, you cannot configure Windows to designate an external disk to be the system disk."