Ok then, I guess I'll just try it and if it fails, I buy the newer 30€ stand that states it supports HDDs up to 8TB. The older one can still be used for the smaller HDDs, like 4TB and less.

What was the symptoms for trying too big HDDs? Were they not detected at all, or did they kinda worked but some problems occurred (even data corruption on the big HDD), or what?

Usually they just can not detect large drives.
You can not make/edit partition or mount the filesystem.

To answer 1, at least with the drives I was looking at they all need to be the same size. Even same drives from different manufacturers might be different sizes, so good idea to check each device and hdd.

For point 2, raid only covers failure of a disk. Individual files you need to handle yourself. There are tools which can perform such checking. Will do a better reply when'd I get home.

Dude, the sizes of those external HDDs reach 10 Tb..
Anyway, if you prefer custom enclosures, ok.

Actually it is only 5TB, because of mirroring.

Second, even merely my GOG game collection is close to 3TB, maybe even more nowadays.

Noo timppu, Wd MyBooks arrive at 10Tb with a single drive.

Ok I thought you were referring to my "external" HDDs (3+3+2+2 = 10). Not sure what you are referring to then.

Probably this one.

Exactly. Btw they're cheaper on Amazon, at least here.

Which is better though? The seagate which is £7 cheaper has 2*5 star reviews, the wd has none.

Also, it's very odd that the external drive is £30-£50 cheaper than the internal drives. Something sounds fishy there.

WD drives have been very reliable in my experience. Not a single one died yet in 6 years.
Btw, a difference is that these WD ones are 5400 rpm.
I reach 70-150 MBps with them, so they're ok for archival purposes.
The only downside (other than the mandatory HW encryption) is that the 8tb v2 becomes hotter than the old 4Tb v1 (48-54° in winter.. I bought a tiny vent for it)

Now that I have checked prices e.g. here (Finland) and Germany (nierle.de), oddly that seems to be the case. I can think of these possible reasons:

1. External USB HDDs just sell so much better than internal HDDs that the volume alone makes them cheaper per unit. Maybe most internal hard drives are SSD then nowadays. (I don't think this would be the only reason, but possibly also affecting it.)

2. Big USB HDDs tend to be those SMR HDDs which are cheaper to make and ok for adding stuff there once in a while, but unsuitable for copying or operating (like compressing) lots of data at a time. I wish there was an easy way to tell if a HDD uses SMR because it would be unsuitable for me (e.g. I sometimes do quite big file operations on external HDDs, e.g. compressing, uncompressing or copying several hundreds of gigabytes of data at a time).

All I know is that apparently the (internal) HDDs which are mentioned to be suitable for NAS-systems, don't have SMR. So at least they should be fine for me. The cheapest 8TB "NAS-HDD" I've seen here costs like 249€, while the cheapest external 8TB USB-HDDs seem to be 199€ or so.

3. At least Finland and possibly Germany (as well as many other European countries) have extra "levy" on certain kinds of media where you can save copyrighting digital media like songs and movies. The justification for that levy is commonly thought to be piracy or even the "fair use" where people copy music, TV programs, movies etc. for their use and copyright holders and artists supposedly lose money, so that money gathered from that levy is somehow channeled to those copyright holders.

So it might be that for some odd reason the external USB HDDs are not hit by that levy, but internal HDDs are. I'll have to check if e.g. the Finnish levy ("hyvitysmaksu" aka "Teosto-maksu") works like that,

EDIT: Hmm, according to the Wikipedia-article that "hyvitysmaksu" for hard drives, DVD-Rs etc. was abolished in 2015... Good if that was the case, it was stupid.

It's indeed annoying, but well, I use double backup copies anyway.
If a controller dies, I'll just connect its drive to sata, reformat (if needed) and copy everything back again.