Grub installation failed when installing ubuntu 14.04 amd64 on usb drive. Laptop is Lenovo Flex14 AMD processor.

[Pam and Geoff Childs]

This is a recent purchase which came (!!!!) with pre-installed Windows 8.1. I am attempting to dual boot with (initially) Ubuntu 14.04 installed on USB3 32GB pen drive. When successful I will then install on hard disk.
I have switched off "Secure Boot" from within windows and I am booting a Ubuntu live disk via a second USB pen drive (works fine).

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: ubiquity 2.18.7
ProcVersionSignature: Ubuntu 3.13.0-24.46-generic 3.13.9
Uname: Linux 3.13.0-24-generic x86_64
ApportVersion: 2.14.1-0ubuntu3
Architecture: amd64
CasperVersion: 1.340
Date: Mon Jul 7 05:30:29 2014
InstallCmdLine: BOOT_IMAGE=/casper/vmlinuz.efi file=/cdrom/preseed/ubuntu.seed boot=casper quiet splash --
LiveMediaBuild: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
ProcEnviron:
 LANGUAGE=en_GB.UTF-8
 PATH=(custom, no user)
 LANG=en_GB.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.