Today, I've re-flashed the BIOS, using a freshly-downloaded FH version. At the end, I selected 'Load Optimised Defaults' but, as you predicted, it didn't make any difference to the outcome. I'm still getting the random FDD accesses and the non-stop HDD accesses.
One thing I've been wary of has been to not remove the pendrive containing the BIOS datafile while still in the BIOS, as it seems to me that because there's no way of properly demounting the pendrive in that situation, there's a risk that either the BIOS data or the file on the pendrive could get corrupted if you simply pulled out the pendrive. So, this time, I left the pendrive in, right up to where I let the system finally boot back into Windows. I then demounted it in Windows in the approved way.
I'm not sure if this is meant to be normal but one rather alarming thing that happens when I boot back into the BIOS each time, having just reflashed the BIOS, is that the PC gets completely turned off, ie. its power supply taken down, and then it powers back up again of its own accord within 4 or 5 secs. So, it's an automatic power-down, rather than a Restart that happens. Is it meant to do that?
Okay, so that's that done again, and no change. I'll now have to see if the different method, of running QFlash off the pendrive itself, rather than relying on the QFlash inside the BIOS chip, will instead work. I'm off now to visit that URL you gave me, where there are descriptions of how to make the USB drive bootable, etc. If, after that, there's still no change in the faults I'm observing, then goodness knows what I'll conclude. Possibly, the Main BIOS chip itself is irrepairably damaged? GGTS have suggested returning the board for testing but that'd involve a massive amount of work on my part and frankly I don't think it'd be worth all the expense; I might just as well try to locate and buy a completely different brand of board.