My first impressions about the causes of the homeserver woes were unfortunately incorrect. The problem was different and worse than I first thought. The disk that failed was not the #4 that was in the swappable position, but the #3. This is very strange, because nothing else had happened to it, not even when the server was turned around. In addition, replacing the memory didn't help at all. So the motherboard was broken and also damaged a hard drive beyond repair. Cause? Most likely the thunderstorm, or more precisely: the electromagnetic nonsense that is generated during a lightning flash. I have experienced this before and it actually continues to amaze me that a 'surge protector' is not built into homes as standard. You can't maintain that it never thunders in the Netherlands, can you? And given the steady increase in electronic gadgets, better security would be very welcome.

Anyway, the end of the story is a new motherboard. Four years ago I had a little more slack than today. Moreover, the excellent motherboard of the time, an Asrock C2750D4I, is almost no longer available, certainly not at a reasonable price. After all, it is an obsolete product....  ;-).
So I chose another motherboard, again one with an onboard CPU, but a lot simpler. With 4 instead of 12 SATA ports. Now one of my HDDs was broken, so 4 is just enough. From now on, they will run in RAID-5, instead of RAID-10.

That RAID thing has cost a lot of headaches. I just printed the complete documentation for the Linux-mdadm module. But I found the redeeming text completely elsewhere, in someone's blog. My old RAID had become inaccessible, because "inactive" as a result of the stopped HDD. No matter what I tried, I couldn't get anything off those hard drives. The solution was (of course) much simpler than you can imagine:
echo active > /sys/block/md0/md/array_state
You just have to put the word "active" in a certain file, and you can access it again. It is true that the RAID is "degraded", but it is easy to read.

Well, with a new, simpler motherboard and 3 instead of 4 HDDs (plus an SSD for the OS, etc.) you also have a file server and certainly a very useful one. The RAID  is converting itself from RAID-10 (4 HDDs) through RAID-0 (2 HDDs) to RAID-5 (3-HDDs). That last step takes a very long time, more than 24 hours, but it is also very complex. My first experiences with this new server are not bad at all. But when you see what such a lightning bolt costs you, you are not in a good mood for a while....