grub installer would not install to usb device

This is the third time I have attempted this with the same result.

I am attempting to install 12.04 to a USB drive so I do not have to alter my Macbook Air's hard drive

ProblemType: Bug
DistroRelease: Ubuntu 12.04
Package: ubiquity 2.10.16
ProcVersionSignature: Ubuntu 3.2.0-23.36-generic-pae 3.2.14
Uname: Linux 3.2.0-23-generic-pae i686
NonfreeKernelModules: wl
ApportVersion: 2.0.1-0ubuntu5
Architecture: i386
CasperVersion: 1.315
Date: Tue May 15 21:08:04 2012
InstallCmdLine: file=/cdrom/preseed/ubuntu.seed boot=casper initrd=/casper/initrd.lz quiet splash -- maybe-ubiquity
LiveMediaBuild: Ubuntu 12.04 LTS "Precise Pangolin" - Release i386 (20120423)
ProcEnviron:
 LANGUAGE=en_US.UTF-8
 TERM=linux
 PATH=(custom, no user)
 LANG=en_US.UTF-8
SourcePackage: grub-installer
UpgradeStatus: No upgrade log present (probably fresh install)