Thank you VERY much for your reply, SgtSixpack! It's like you read my mind; overnight I got to thinking about having given up on creating bootable media, and resolved to find a "for dummies guide" and give it my best again. One of these weeks.
Sorry I'm afraid I wasn't clear enough. I was simply explaining that my need for bootable diagnostic media isn't simply "a case of needing to run tests on a...single drive pc". I was trying to explain that bootable media can be required to run diagnostics on even a storage (non-system) HDD, because over the years I've found that often (on some systems) "Windows won't boot if one of it's NON-system HDD has issues calling for diagnostics" (by which I meant bad sectors, which I can only thus identify/find with bootable media diagnostics).
As for my problem of "one of my PC's can't run anything newer than 8.1, the other can't run anything newer than 7"; I've accepted it. With the 8.1 machine for example, I gave it every spare minute I had all summer 2016; one guy on a forum applauded me for my persistence and thoroughness. (These are pretty old PC's; my finances are tight.) With the other PC, maybe I could get it to go beyond 7, I honestly didn't give it nearly as much exhaustive effort, but I just don't think it's worth it given how overwhelmingly busy I am. I'm prepared to put it out to pasture when Win7 goes EOL.
Edit: I just phoned WD Tech Support. They said their bootable (DOS) hard drive diagnostics supports not only the the WD lines of Red and Gold hard drives, but also supports HGST hard drives. Oh, I am undeterred from my resolve to make my own bootable media with HD Tune on it, particularly since, as I mentioned on another thread "I found that unlike some manufacturer's drive diagnostics, it reports ANY errors (apparently some manufacturer only report errors above some amount)."