The consensus for installing an 100% independent Linux on an external HDD, without having any boot record etc interference with any other OSes on any other HDDs, is to unplug all your HDDs, plug in only the live CD/USB and the destination HDD or flash drive, and install.
Because every time I see a guide for doing it without unplugging the rest of the HDDs, it is riddled with comments of how it backfired (exhibit A, exhibit B, plus I tried it a couple of times and gave myself huge headaches and a lot of work afterwards).
I have a Windows laptop that I can't open up, and I don't have access to any PCs. So I will ask, clearly how else can I, what's the actual way to, easily, with minimal risk of screwing up, with minimal steps, install Ubuntu without corrupting my main system or leaving any other traces or dependencies in it?
I would love it if there just was a button for "install everything to this HDD clearly labeled as 'MY External HDD'". OR to download a 16GB pre-installed sector-by-sector Ubuntu HDD image that I could "recover" to my external HDD and then resize the data partition accordingly. Because that guarantees not messing anything up big time, and it also doesn't require any OS restarts or ISO boots for the process.