time machine is a great way but if you are not running it there is another option. use your disk utility, select the drive your want to back up and click restore. select the drive you want to back up as source, and your external hard drive as destination, then hit restore again. Disk utility will then make a bootable copy of your drive. Depending on the size of your drive this may take a while. Or then again, you could just activate time machine and select your external (empty same size or bigger) hard drive as the designated drive.
All the best,
Wolf