Welcome to LQ.
What you want to do is turn your laptop into a file server
- which is easier than the letter of what you have asked for. Also more flexible and useful.
rt-click on the folder you want to share, select "sharing options" and follow instructions.
If you only plan to use this with computers that are not on a network, nfs sharing using a crossover ethernet cable will probably work well for you.
There are projects to allow gnu/linux computers to act as UMS devices when plugged into other computers by usb cable. Normally for embedded linux as in phones and pdas. eg.
https://wiki.ubuntu.com/MobileAndEmbedded/USBClient
I don't know of any special headway. linux is just good at server stuff. So if you really want a usb connection to serve files, you are looking for IP over USB.
http://www.linux-usb.org/FAQ.html
http://usbip.sourceforge.net/
http://www.linux-usb.org/usbnet/