Hey there,
A small update. As you've seen I have quite a bunch of HDDs connected to the computer. I've been going crazy today about it, as multiple drives randomly kept on disconnecting, reconnecting, disconnecting, reconnecting...And no reboot, neither of the computer, nor the HDD cases seemed to solve the problem.
That's why I now disabled CSM Support in my BIOS and now I had a boot logo and the drives ordered "correctly" for the first time. The system is stable too. That's why I opened this thread asking for CSM recommendations to verify my assumption.
However, as this was going on for the past 6 or so hours, I'm still a bit skeptic.
In general, if anybody is familiar with multiple HDDs in 4-/8- and 10-bays, I'd soak up any insight you might have, as it's pretty annoying to reboot the computer every 5 minutes due to an HDD I/O error or similar (All bays are connected through USB 3.0 or Type-C directly to the motherboard).
It's a little bit off topic, but my guess is that the no-boot-logo/yes-boot-logo situation, drives and their partitions + CSM support are all tangled up together.
Have a great day wherever you are. I keep praying that there won't be the next HDD failure just around the corner ^^
EDIT: Ah wow! Awesome! Now indeed all HDDs work, but they're all not mounted where they used to be What the actual heck is going on
i.E., what used to be /mnt/Volume6/... is now /mnt/Volume2/...
and /mnt/Volume3/... is now /mnt/Volume4/... and so on
Meaning nothing is where it used to be, which is pretty much the worst case scenario.
Holy mother of God... Something in my setup is just very, very wrong