Sharing USB Drive - XP/Vista

I hope someone can help me. Is there a reason that a USB drive can't go from a Vista system to an XP system? My primary system is Vista and I just bought a net book (XP) and I wanted to share files (since my net book does not have a CD drive) so I purchased a 650 g Hitachi USB drive. I was able to download the files to the Hitachi drive and upload the files to the net book, but now I am trying to backup my Vista system and the computer does not even register the USB drive at all. Can someone please help me.
Thanks