Accurately describing what GRUB does gets into a lot of technical details, so some of this will only be half-truths in the pursuit of making it understandable. There's some background information that you may already know all or some of, but i'll try to give a high level description of everything just in case. I don't want to come off as condescending - I'm just not sure what all you already know
When you first turn on your computer, control first goes to some code called the BIOS that sits on a chip integrated into the motherboard (The motherboard being the piece of hardware that coordinates the communications between all the other hardware - the processor, hard drive, cd-rom, etc.). The BIOS will usually do some checks to make sure various other pieces of hardware are working correctly, and then passes off control to a
bootloader.
The
bootloader is software that sits on part of your hard drive that is set aside for this specific purpose, called the
Master Boot Record (or MBR). Your windows XP machine has a bootloader installed right now (called NTLDR), which once the computer powers up, tells it to run windows. GRUB is another bootloader, but it does a little more than NTLDR. Instead of just starting windows, it allows you to select which operating system you want to run. Like NTLDR, it's installed on the MBR of a hard drive.
Both your internal and external hard drive have an MBR. Right now, your internal hard drive has NTLDR on the MBR, and most likely the MBR on your external hard drive is empty.
If you want to go the multiple-OS route, this all leaves you with two options:
1.
Overwrite NTLDR on your internal hard drive with GRUB. You can then install linux on your external hard drive, and GRUB will let you choose between running windows on your internal hard drive, or linux on your external. Depending on the distribution, you may need to configure GRUB by hand to tell it where your operating systems are - which then gets into a discussion on hard disk partitioning which I won't get into until necessary.
2.
Leave NTLDR alone, and install GRUB on the MBR of your external drive. This has the benefit of not touching anything on your internal hard drive. The downside is, you'll need to tell the BIOS that you want to use the MBR on the external hard drive isntead of the internal one. How you do this depends on which BIOS your system is running, but it's usually a matter of hitting a specific key when you first turn on the computer, which brings you into the BIOS option menu. Somewhere, there should be an option along the lines of "boot from" or "select bootable device", where you can select which MBR the BIOS will pass control to on startup. The exact steps of doing this are dependent on which BIOS your system uses.
The easiest to set up would still be to dedicate the entire internal hard drive to linux. GRUB will still be used, but it will function more like NTLDR in that it will just start linux, and the installer of whichever distribution you use should be able to automatically configure GRUB for you. You won't need to change anything in the BIOS, because you will be using the same MBR as you are today, it will just hold GRUB instead of NTLDR.
Reinstalling windows XP will automatically overwrite the MBR with NTLDR and get you back to a windows-only system, so you always have a safety blanket if things go wrong.
This is a lot of information, but hopefully it's relatively understandable, and knowing some of the details should make things more comfortable as you're doing the set up. If this has been at all helpful, I'd certainly be willing to do something similar for hard disk partitioning, since that seemed to be a source of apprehension.
If not, let me know and I'll stop cluttering the thread with this silliness ;D
I think the tools available now are much, much safer, but you'll want to have that security blanket regardless.
