Your desired result is important. I do not think "disk to disk" is what you are wanting since you just seem to be cloning your drive.
If you just want to copy the files to the external, you can just copy them. They may not all copy and this method would just allow for you to access the files that did copy, but not restore your system.
If you want to make a backup image, with which you can recover your install if the primary hard drive fails, you need an image. Also keep in mind, some images can be accessed without having to restore them. I know the Windows 7 Backup Images can be done this way. This method would create a folder on your external drive and just use the space it needs.