You want to back up. You could go done the clone your hard drive to an external to make it bootable you can use either Carbon Copy Cloner or SuperDuper. To make a quick & dirty photo backup then mount you external onto your desktop and just drag the photos to that other drive. When you do the dragging (while leaving the Finder window still open) a little + sign will go over the files when dragging from drive to drive. This is an indication that OS X just copies files to two different hard drives and does not move the files. Try it with any file and you see what I talking about.