install in a md (software raid) disk

[Pandaroux]

Install grub on a MD disk (software raid) fail

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: ubiquity 2.20.0
ProcVersionSignature: Ubuntu 3.16.0-23.31-generic 3.16.4
Uname: Linux 3.16.0-23-generic x86_64
ApportVersion: 2.14.7-0ubuntu8
Architecture: amd64
CasperVersion: 1.345
Date: Tue Mar 24 16:45:03 2015
InstallCmdLine: noprompt cdrom-detect/try-usb=true file=/cdrom/preseed/ubuntu-gnome.seed boot=casper initrd=/casper/initrd.lz quiet splash -- debian-installer/language=fr keyboard-configuration/layoutcode?=fr keyboard-configuration/variantcode?=oss
LiveMediaBuild: Ubuntu-GNOME 14.10 "Utopic Unicorn" - Release amd64 (20141022.1)
ProcEnviron:
 LANGUAGE=fr_BE.UTF-8
 PATH=(custom, no user)
 LANG=fr_BE.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)