I agree with Riptides. 3 copies. 2 different types of media. 1 in a different location. I have a friend who backed up his pictures from a laptop to a USB drive on his desk. He was robbed and they took his laptop and the USB drive he backed up too.
Also I've talke to people who backe up to a drive. But delete the pics from their laptop. USB hard drives fail at the same rate if not higher than internal drives. I know I've killed 2 myself by accident. One got knocked off my desk to the floor and the other I dropped while moving it to another room.