Success at last! There's a video that describes the problem and gives a fix: YouTube. This method worked like a charm: no errors, no reboot (and no re-install of W10). Windows 10 can now see anything attached to HDDn.

Since all owners of older, high-end Dell's are in the same position, I cannot imagine why Dell didn't just issue a Windows 10 driver update. At the least they could have put out an advisory.

Interestingly, the successful Windows 10 driver seems to be just the standard Windows 7 driver from 2013! (For W10 Microsoft has thrown in something else but fortunately this is included in the driver download.)

It has taken months of almost continuous research & effort to make dual boot work properly with a hard disk and an SSD. And in the end the fix took just 10 minutes. That's two months of my life I'll never get back! I don't know whether I feel angry, relieved or elated. Probably all three. Anyway, it's the pub and a pint tonight..