If you are not using the machine for other things, just for HA, ditch one of the hard drives, keep just one in it.
Install Debian → How To Install Debian 11 (Bullseye) with Screenshots – TecAdmin
Install HA → https://github.com/home-assistant/supervised-installer
Done.
Thanks for the links. Very helpful.
The only issue that I ran into is that I had to create a new user with Admin rights because I couldn’t change the default user rights to sudo.
Will this work on Ubuntu too?
Ubuntu isn’t a supported OS for a Supervised install. Check this guide for a more detailed walk through of the install process.
I’ve started with HA on Ubuntu before, had to change to Debian, because I had some issues. Can’t remember anymore exactly what. I do remember the important part - it did not worked perfectly 
I got mine working now on a Beelink T4 mini. I installed the Generic x86-64 image and here is how i got it working.
Download and flash Ubuntu image on a flash drive or SD card. Any ubuntu will do. I used 20.04
Copy the downloaded haos_generic-x86-64-6.6.img.xz image into another flash drive or SD card
Boot with the ubuntu flash drive but select try ubuntu without installing
When ubuntu is booted, Insert the second flash drive
Open the flash drive and right-click on the HAOS image, open with disk image writer
select the T4 mini disk (or NUC or your laptop) as the destination and restore
I just bought a Dell OptiPlex 3040 i3-6100T Micro (SFF).
Will Home Assistant run on this as a stand-alone server?
Yes it’ll be great. How much RAM?
It should work. Just remember to Make UEFI Boot mode is enabled and Disable Secure Boot.
You can follow my guide above if you want to use the entire system for HA OS
Sorry to everyone that responded to my question, I’ve been having too much fun with this setup.
For RAM I just upgraded the RAM from 8GB (2x4GB sticks) to 16GB (2x8GB sticks).
For now, I am running Home Assistant Core now I am thinking of moving up to Home Assistant Supervisor.
I am so glad that I tried this software again, it was 3 yrs ago when I tried it for the first time. I am so glad that my IT education has paid off…lol
Hmm… The first post I read on this forum starts with a lengthy quarrel, then the tread gets hi-jacked before being hi-jacked again… 
This is by no means the worst thread.
Anyway, you are new. How can we help you friend?
Welcome to the forum.
Given the thread you have posted in, I’m guessing this is the best place to start.
The correct answer to this is that it will depend on the load you are going to submit your system to. A simple HA install, to tend to a regular home may have 50 different devices and run a couple integrations or addons and run fine on a Rpi 3, but if you just add another service, like plex for an instance, as you mentioned, it will not work at all. This because a plex installation, depending on the type of content you will serve, will be very CPU intensive and that is not the purpose of a RPi 3. If you plan to stream 4K video, let’s say to 2 different TVs at the same time, in a household, what is not something so difficult, this means that not even a maxed out RPi 4, with SSD and 8 GB of RAM will do, since this is not the intention of this also. You would need a more powerful CPU for that.
The answer to this question is you need to plan and bring together the hardware needed to support your designs.
It is important to positively insure that, for most purposes, a RPi 3 is more than enough to admin a regular household with HA with plenty of power. But if you plan to add something, it is important to check the minimum requirements for this. In the case of the plex server, in their site they specify that a minimum of an Intel i3 and 2 GB of RAM is required, so I would say that your computer, that runs or a celeron or a pentium CPU does not qualify. I would not bet on that to run a plex server as an addon to a HA installation.
The idea to run anything in a RPi system, if you ask me, is to have something that will do the job, it is a ARM Quad Core with 1 GB or RAM, plenty of USB Ports, Ethernet Port, Wifi and Bluetooth, not to mention plenty of in and out ports so you may do as you wish, that will not consume a lot of energy, that is somewhat a industrial style device - a RPi is constructed so you may play and test things. That is, undoubtedly, a very capable little computer.
Thanks for this. I was able to install in on a Beelink U59 Pro. I have a question, this device has an optional 2.5 bay where we can install a 2.5 harddrive as storage. Will this version of HA install detect the extra drive??
If you are running HAOS the official answer is ‘no’. But using the web terminal & ssh addon it is certainly possible.
Is the built in bluetooth adapter of the BT4 recognized by HA?
There is a Bluetooth device detected. I am not sure if this is the onboard for BT4
I got one beelink T4, but after flashing the emmc, the drive does not appear in the bios as boot option. Did you face this issue? How can I fix the boot?
I did not encounter this. Not sure if this is related but did you configure your BIOS as per HA guide?
https://www.home-assistant.io/installation/generic-x86-64
