Anyhoo, my main takeaway lesson, from this and the "phantom disk" thread, is to beware of false economy.
I had a decent Plex server running for 3 to 4 years using >10 yo bits which was fine. However, when this started to go wrong, penny-pinching made me invest in a large number of ineffective remedies to try and rescue what I had. I now have a decent Plex server on a modern(ish) AM4 board with a Ryzen 3, 16GB of DDR4, enough disk space for the time being, and a new (very old design) Coolermaster case. This should last a while. The downside is that the ancient case design means that the cabling is a total rat's nest. Plus I have a semi-operational SATA card and no obvious way to expand.
If I'd just bitten the bullet straight away, I could have bought something like the much better
MSI PRO B550M-VC WiFi motherboards, which has 8 SATA ports and 2 M2 slots, all of which can be used, and a fabulous case like a
Fractal Design Meshify 2. When I include things like cables, brackets, case fans (included with the Fractal Design case) etc etc , I'd probably still have change from what I spent.
*Also, I wouldn't have tried to cram everything into the original case, causing a plastic tab to snap off one the SATA drive data connectors so that I now have one drive with a cable that has to stay in place and not be knocked or shaken or the drive stops working!