Yes, you must have your flash memory stick already plugged in before you start the computer up so it will be detected during P.O.S.T. and registered in the BIOS.
how do you boot from the flash drive? do you plug it in and start the computer with the USB drive in or can you start from Windows/Mac OS X?
There are at least three different ways that I use, and probably others can tell you more ways to boot a USB disk.
1. If you don't have GRUB installed to MBR in your computer, another way to boot your USB flash memory device is by pressing a key as your computer boots to bring up a menu from your BIOS this way, How I boot from my BIOS with my F12 key (or F8 key in most PCs).
2. If you still have Legacy GRUB - You can use GRUB's Command Line Interface to boot your flash memory stick manually.
To make it more automatic you can add a new entry to the bottom of your internal drive's Ubuntu /boot/grub/menu.lst file which will chainload the MBR or the boot sector in the USB flash memory stick.
3. If you're using GRUB2 - Just boot into your regular Ubuntu installation inside your computer and run 'sudo grub-mkconfig -o /boot/grub/grub.cfg', and it will automatically add an entry for the USB Ubuntu auxilliary operating system. Then boot your auxlilliary Ubuntu USB and run 'sudo grub-mkconfig -o /boot/grub/grub.cfg' for an emergency boot entry in your USB for your main (internal) Ubuntu installation.
title Hardy Heron USB Stick MBR Boot