As I said in my last post, there are several parts to the Grub bootloader. If you boot Ubuntu and go to the directory /boot/grub/ you will see a number of files. There will be a stage1 file which is the only file in the mbr (check the properties, 512bytes). This is not enough to boot any system so it has to go to stage2 (which contains the menu you see when booting) and sometimes stage1.5 before that for the particular type of filesystem. If you have most of your Grub boot files on a hardrive that is not plugged in it obviously won't work.
You can boot Linux from xp or have a separate /boot directory on the 80GB internal drive but this second option would be difficult as you already have xp installed there. I'm not sure how or if this could be done. Here's a link explaining how to boot Linux from XP.
I haven't tried this myself so not sure how well it works. I've come across a number of sites explaining how to boot Linux from windows so it can be done. From what I have seen, it seems a lot easier to boot windows from Linux (Grub) than the reverse. If you plan to stick with xp and just play with Ubuntu or Linux booting from xp would probably be the way to go.