USB drive boot manager

Hi /prog/

I have windows installed to one HDD, and *nix to another. ATM i use the BIOS to switch between drives at boot. This is because I want each drive to remain bootable on its own [eg if i brick one of the two OSes]. Obviously this isn't very efficient.

I'd like to have a boot manager installed on a USB drive to select between which to boot, but I want to avoid doing anything that modifies the drives so that they become unbootable without the USB stick either.

Can you point me in the direction of some software that can do this? I've looked at Plop, YUMI, GRUB and LILO but i'm not really sure which best suits my needs, and if its possible to use them in the way i describe [from USB stick, with USB keyboard and without modifying their MBR etc].

It'll probably take you less than an hour to learn how to write your own chainloader. All it has to do is relocate itself, wait for a key to be pressed, and load the first sector of the appropriate drive and jump into it. Any existing solutions are going to be much more than you need.