CentOS

I have just done a reasonably large OS upgrade ...

As jlehtone asked, upgraded from what?

Did you try Ctrl-Alt-F2 as jlehtone suggested?

I think it will only work with the running kernel.
So if you installed it with old kernel it would not work.
You could try to boot from the new kernel, then crtl+alt+F2
should give you console login where you can install it.

Another thing to consider is that fdisk etc. will show no partition. If someone else is administrating this PC, they may think there's nothing on the disk and then creating a new partition and file system, i.e. damaging any data. Is this a VM? (- I don't remember seeing any physical disks with these...

AFAIK that is ok.
But, when you use sdb there's no need to partition it (normally, when you
partition it creates /dev/sdb1 which should then be used when creating the
file system). And the limit for fdisk is 2TB, not 2GB. So the instructions in
the link isn't the very best.

From what you write it sounds like you haven't created partition(s) and file system(s)?
What's the output from this:
fdisk -l
And how did you mount it?

Where it should be mounted depends on what you're using it for.
Are you sure it's sdb and not sdb1?
When you reinstall the OS, you have to remount it, either manually or add it to /etc/fstab.

"Note that cPanel will not support CentOS Stream."
https://blog.cpanel.com/centos-8-end-of ... ouncement/

It exists for CentOS 8 - maybe there's no version for Stream:
$ yum whatprovides inxi
inxi-3.3.02-1.el8.noarch : A full featured system information script
Repo : epel
Matched from:
Provide : inxi = 3.3.02-1.el8
$ rpm -qa|grep epel
epel-release-8-10.el8.noarch

Is this any help:
https://wiki.centos.org/HowTos/InstallFromUSBkey