This project for me is more useful than it first appears, I use this mouse all the time and all though it was quite fiddly to make It was worth the effort.
Now why would I want a mouse with a USB port hanging out the side? The main reason is that I do a lot of computer aided drawing on a lap top which is impossible to do with a track pad and I share and collaborate with students who have all manner of different shapes and sizes of USB drives.
So whats the problem? Well my mac book has the USB ports so close together that it is often not possible to have two things plugged in at once, It drives me mental, and I didn't really want to have to use a USB hub, as it just another item I have to carry around and get lost.
Also with its internal drive It also saves me carrying around a USB, and it useful for when I use a desktop machine as I don't have to scratch around under the desk to find a working USB port.
The instructable has lots of notes attached to the photos