Réinstaller grub efi aprés une install débian foireuse...
0) récupérer refind et booter dessus (utiliser drivedroid si vous avez un android rooté :x )
http://www.rodsbooks.com/refind/getting.html

1)
[ -d /sys/firmware/efi ] && echo "EFI boot on HDD" || echo "Legacy boot on HDD"
should return "EFI boot on HDD".

2) en ligne de commande et en root (donc sudo -i)
(si sda1 est votre partition EFI)
mount /dev/sda1 /boot/efi

apt-get install --reinstall grub-efi

grub-install /dev/sda

update-grub

You should check afterwards that:

1. the bootloader is existing in /boot/efi/EFI/debian/grubx64.efi and
   file /boot/efi/EFI/debian/grubx64.efi

/boot/efi/EFI/debian/grubx64.efi: PE32+ executable (EFI application) x86-64 (stripped to external PDB), for MS Windows

2. the nvram entry was properly created.
   efibootmgr --verbose | grep debian

A TESTER D'URGENCE