grub-installer fatal error

[silvio]

installing on a ssd from a fat32 partition on an external usb hdd

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubiquity 2.17.9 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 3.13.0-17.37-generic 3.13.6
Uname: Linux 3.13.0-17-generic x86_64
ApportVersion: 2.13.3-0ubuntu1
Architecture: amd64
CasperVersion: 1.338
Date: Sun Mar 16 19:12:24 2014
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper only-ubiquity quiet splash --
LiveMediaBuild: Ubuntu 14.04 LTS "Trusty Tahr" - Alpha amd64 (20140316)
ProcEnviron:
 LANGUAGE=fr_FR.UTF-8
 TERM=linux
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=fr_FR.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)