are you wanting to COPY your files, or clone your hard drive... there is a big difference. If you want to copy your back files, media, docs, etc... then just drag and drop the files onto the external, but if you want to boot from it. that would be different.
Does your laptop have a recovery partition on it? Why not just save the stuff you want, then let the recovery bring the laptop back to factory specs without having to go through all the mess of an external boot drive? This option is usually much easier.