Happy new year to you and thank you for answering so quickly.
The problem was i hadn't find the way to authorise the mbr or the uefi
writing in the bios.
Now, all works fine (Xubuntu and win8.1), but there is a message saying the
laptop is unprotected just before the GRUB prompt.
However, this
Your EFI system partition is corrupt. You will need to repair it either
using dosfsck from linux, or chkdsk/scandisk from Windows.
** Changed in: grub-installer (Ubuntu)
Status: New = Invalid
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Bugs, which is
