as grub loads up, b4 u enter the os, ie press ESC while grub is loading, then press "c", this will drop you into a command line
type "root ("
then press tab
this will give you a list of devices grub can use, i'm not sure what your usb drive would be called, possibly hd1, but that could also be your second HDD, this all depends on your bios settings,
once you got your root command which should look something like
*# is replacing the letters i'm not sure about
type the next command which is kernel, now this one is fairly strait forward, it's gonna be
"kernel /boot/vmlinuz-2.6.[TAB] root=/dev/sd## splash quiet ro --"
then press enter
tab will fill in the rest of the filename, those two hash's represent the drive name as recignised by ubuntu, you'll want it to be the device name of your usb drive.
next command is even more simple, initrd.
as before tab will fill in the rest of the file name
now just type boot, hit enter and your away, keep note of what you put in so that you can change you "menu.lst" file later, this is stored in /boot/grub/menu.lst
and then you should have a fully functional usb ubuntu drive XD