Yeah unfortunately it's not uncommon for NIC hardware to have proprietary drivers (although wifi is often worse). Having said that, I just did a quick bit of research for you.

According to HP (and a newer version here, plus a few other sources that seem to confirm they used the same NIC across different versions) the "HP ProLiant MicroServer" uses a "Embedded NC107i PCI Express Gigabit Ethernet Server Adapter". A bit more searching suggests that apparently the NC107i PCIe NIC is based on a Broadcom BCM5723 chipset. According to Debian (what both TurnKey and Proxmox are based on), that chipset uses the 't3' driver. And the 't3' driver (t3.ko) is bundled in the kernel.

So my quick searching suggests that the NIC should "just work".... Fingers crossed!

If for some reason it doesn't, then before you start trying to install drivers from all over the internet, I urge you to check the "non-free" firmware available from Debian. You'll need to download the deb file onto a USB, then manually mount it and install it. But lets cross that bride if/when we get to it... :)

It's definitely an I/O error and the note about "media error" certainly does suggest a HDD issue. According to what you've posted, it's having issues with whatever drive is connected as 'sdb'. Having said that, there is also a possibility that it's a symptom of a motherboard, HDD (or HDD power) cable, RAM and/or power supply issue(s). The fact that you say the error repeats over and over does at least suggest that it's not an intermittent issue (which are the worst to troubleshoot).

First up, I recommend some basic troubleshooting. Ideally, do that within the existing system. That is if you can - if it's just spamming your terminal too much, then you may need to boot from a live USB.

First thing to do is work out what drive 'sdb' is. This should help (all commands are assuming you're root, if not 'sudo su' first, or prefix each command with 'sudo'):

hdparm -I /dev/sdb

Hopefully that at least gives you enough info to recognise what the drive is. If the system isn't really usable (because of the errors spamming the terminal), then take note of the info so you can find it using a Live USB (read down for more info on that). If it is usable, then you could try installing smartmotools and doing a SMART scan of the drive:

apt update
apt install -y smartmontools
smartctl -a /dev/sdb

That should show all the SMART data about the drive and give you a clue as to the likelihood of whether it's failing. If that shows something like this:

SMART support is: Available - device has SMART capability.
SMART support is: Disabled

Then run this instead:

smartctl -s on -a /dev/sdb

If the system isn't usable, then you'll need to try with a Live USB (i.e. a Linux Live ISO "burned" to a USB). Seeing as Proxmox is based on Debian, then I'd recommend just a Debian Live image. If you don't need any "non-free" drivers, then I'd recommend either the lxde one, or the lxqt one from here. Otherwise, the current ones with "non-free" drivers are here (for future readers, double check for newer releases here).

FWIW, if a TurnKey ISO boots ok in Live mode on your hardware and has networking, then you could even use that (download Core if you don't already have a TurnKey ISO)! Any other Debian or Ubuntu based live USD would probably be fine too. If you're on Windows, then I suggest Rufus to "burn" it to a USB (don't use NetBootIn!).

Once you've booted into a live system, If it doesn't have smartmontools installed already (which is likely), install it as per above. Note though, that under the live system, the problematic drive may no longer be 'sdb'. So first check to see if it is (compare with the info from earlier):

hdparm -I /dev/sdb

If that's not it, then you'll need to try to work out what it is. This should give you all the options:

ls /dev/sd* | grep -vE [0-9]

Then use 'hdparm' as above, trying each one until you find it. Then use smartctl on it.

Thanks for sharing about Ventoy. I'd never come across that before and it looks cool!

Also, I didn't realise that your Proxmox issue was getting the install medium (i.e. USB) to boot?! I'm guessing that you are a Windows user? If you're a Linux or Mac OSX user, you can simply write the ISO to a USB using 'dd' (a low-level CLI tool) and it should "just work". For future reference, if you are on Windows, rufus is a good option (although doesn't look at cool as Ventoy). It may be fixed now, but I recall NetBootIn is (was?) known to have issues with Debian.

Also, FWIW, the only difference between the "official" and "non-free" Debian installs is that the latter has the "non-free" Debian apt repo enabled by default and includes all of the non-free Debian firmware packages pre-installed. Usually, the only drivers likely to cause issues (on a server system) are network drivers. If you do a bit of research on the hardware you have and work out what the chipset is, then you can download the relevant driver package and install it and you should be good to go... If you need a hand with that, please let me know and I can help out.

Awesome to hear that you now have it up and running! :)

Re Apache, as you're likely aware, you can configre Apache within Webmin. However, I personally find Webmin Apache config a bit confusing and frustrating. IMO manually editing the Apache config files (in /etc/apache2) and using the commandline utilities is much more straight forward and intuitive (YMMV as I am a CLI lover...).

If you do go that way, then the upstream Apache docs are pretty good IMO. The only thing to keep in mind is that instead of being a single monolithic config file, Debian splits the config up. E.g. "virtual host" sites go in /etc/apache2/sites-available/SITENAME.conf and ports are configured in /etc/apache2/ports.conf, etc. Separate virtual hosts files are then enabled/disabled via 'a2ensite SITENAME'/'a2dissite SITENAME' respectively. Note that config changes are only applied on restart:

systemctl restart apache2

Obviously if you're happy with Webmin, don't let me get in your way! :) And ask if you need a hand (although I may not be super helpful with Webmin...).