1. Get yourself a slim USB drive that doesn't need any power. I use the Western Digital. They are small and go up to 320GB.
2. Add some velcro to the back of the drive and then to the back of your laptop. Ideally skew it to the side of the computer that has the USB/Firewire ports so you can use the short cable that comes with the drive.
3. Velcro on and go.
This would also work with a USB powered USB hub.