How to install windows 2003 server in external hard disk

I've never done this in practice, but in theory it should work if you add the drivers for it to your setup. So that you can see the drive during install.
If that doesn't work, I'd think the only way to do it would be to copy an installed system over to it, and insert a working bootsector to make it bootable. Don't have instructions for that though.

Alternatively, you can install linux on an usb drive, and run a 2003 server from vmware or similar