Thank you for your answer.
I installed windows 8 using the fujitsu recovery DVD I got when buying the laptop (2 weeks ago).
I am quite sure it is in uefi mode.
I don't know what this means exactly: "did you mount any of the partitionss in life mode before starting the install process". I didn't do anything special.
I don't know if the USB starts in uefi mode. As I told before in the boot menu I choose USB HDD (it doesn't say if it is uefi or not). But when booting the usb I get a black screen with in the left top corner something like:
Try ubuntu without installing
It seems that this means that the boot was in uefi, or not ?
Thank you for your kind help.