installaion 14.04.3 on external drive

[Bougron]

while installing
 "Le paquet grub-efi-amd-signed n'a pas pas pu être mandaté dans /target.
En l'absence du programme du démarrage de grub, le système installé ne pourra pas démarré"

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubiquity 2.18.8.9
ProcVersionSignature: Ubuntu 3.19.0-25.26~14.04.1-generic 3.19.8-ckt2
Uname: Linux 3.19.0-25-generic x86_64
ApportVersion: 2.14.1-0ubuntu3.11
Architecture: amd64
CasperVersion: 1.340.2
Date: Mon Sep 14 12:14:17 2015
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash --
LiveMediaBuild: Ubuntu 14.04.3 LTS "Trusty Tahr" - Beta amd64 (20150805)
ProcEnviron:
 LANGUAGE=fr_FR.UTF-8
 PATH=(custom, no user)
 LANG=fr_FR.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)

[Bougron (francis-bougron)]

 i have decided to erase ubuntu 15.04 on partition sb9 by ubuntu 14.04.3 with option
'autre chose' and without demouting the sdb and formating sdb9