[CentOS] Now you did it Olly

Mon Jul 28 11:49:56 UTC 2008
D Steward <dsteward at internode.on.net>

Ah sorry, you meant bootloader not MBR.
You'll probably have to reinstall grub.
Have you been using this guide?
http://www.ubuntugeek.com/a-much-easier-way-to-install-ubuntu-on-a-usb-device-stick-or-hd.html