<p>As a practical matter, one useful thing to do when you want to be sure not to lose papers is to send them to yourself when you hand them in; that way they are prety much permanently in your own email system and you can retrieve them if you ever need them. It is also a good idea to save on as many kind of disks as you have and keep them in a safe place--and keep track of what that place is. When you say you cannot find a copy on your computer or disk it isn't clear whether you neglected to save it or the comptuer ate your homework:)</p>