I used the article at http://www.thewindowsclub.com/windows-10-media-creation-tool-create-installation-media-upgrade and because I needed it to repair another W10 laptop I chose that option.
As you asked about download and not installing which is an entirely different topic, you could get the .iso to any drive of your choice.
Now if you want to install W10, you can do that too. There are numerous articles about dual boots and about Windows To Go. Read from https://www.google.com/#q=windows+to+go+windows+10
Frankly that system is not one I'd go near.
Is it possible to download windows 10 onto a portable hard drive rather than your computers hard drive please???

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