How does portable Windows licensing work?

I don't have enough room available on my Mac to be able to Bootcamp it properly, so I've installed Windows 10 as a portable install on a USB flash disk using Rufus. It runs smoothly and it's fine for what I want - I rarely need to use Windows.

However, I'm keen to take advantage of the free upgrade for a Windows 7 license to Windows 10 (a Win 7 Pro licence is around £20 at the moment) whilst the offer is still on for the next week.

How would this license work with my portable install? I'd like to be able to use my USB stick with several different computers (including my Mac). Would this be possible? Also, how would upgrading the USB drive etc. in future affect the license state?

Any help is really appreciated!

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