Grub install fails on installing lubuntu on external USB HD

[Ric Flomag]

Circumstances:

* Installing Lubuntu on an external USB HD, using a Lubuntu live USB, 64bits.
* Before i started, I had removed the internal HD from the computer, as a safety measure.
* At the beginning of the install procedure, I choose the manual partition set up option, and i don't modify the current partition scheme (one root partition + swap), without formatting the root partition, but overwriting a previous ubuntu 12.04 (I want to keep my user folder).
* without internet connection to speed up the process and install updates and language files after.

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: ubiquity 2.10.16 [modified: lib/partman/automatically_partition/question]
ProcVersionSignature: Ubuntu 3.2.0-23.36-generic 3.2.14
Uname: Linux 3.2.0-23-generic x86_64
ApportVersion: 2.0.1-0ubuntu5
Architecture: amd64
CasperVersion: 1.315
Date: Sun May 13 20:55:13 2012
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz file=/cdrom/preseed/lubuntu.seed boot=casper quiet splash --
LiveMediaBuild: Lubuntu 12.04 "Precise Pangolin" - Release amd64 (20120423)
ProcEnviron:
 LANGUAGE=es_DO.UTF-8
 TERM=unknown
 PATH=(custom, no user)
 LANG=es_DO.UTF-8
 SHELL=/bin/bash
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.