ORLLY?
Is it legal to make a copy of a game I own?
Yes. U.S. Copyright laws allow you to make one backup copy of software you own. However, that backup copy is "tied" to your original. If you sell or give away your original software, you must destroy your backup copy or package it with the orignal when you sell or give it away. Likewise, you may not sell or give away your backup copy without the original.
Of course, with the advent of the Digital Millennium Copyright Act (DMCA), breaking the copyright protection on a piece of software you own has been made a crime in many cases, so if the software you are backing up implements copyright protection, it may also be illegal.