Normally the copyright protection is in place to prevent exactly what you want to do.
However, according to the US Courts, you have the right to make back-up copies of copyrighted disks for your own use. Its just when you "loan" out your back-ups, rent them or sell them that you run into troubles. But if its for your own use and you only make one back-up, then you can make a copy.
However, some commercial disks contain more information than the capacity of retail blank disks. Because of this, you will need a helper program to "shrink" the content to fit your back-up disk.
This freeware program can assist you in making your back-up.