Hey Shockwave, for your first question. I found an easy way that has always work for me and very easy installing Batocera on a HDD, using a Windows or Linux machine.
First download Etcher (win or Linux, have not tried Mac but I recommend do it on a Win machine)
Connect the HDD you want Batocera on as a 2nd storage for your computer.
Run Etcher, by default, Etcher will not allow you to install any ISO into anything else that is not a Memory Stick, but if you click on the gear icon on the top right corner it will open the settings menu, on the Advanced section check the box that says Unsafe Mode, and uncheck the one that says Validate write on success and click on the back button ( don't worry I have done this many time and it had never fail once).
Then select the Batocera ISO image on the first step, now when it ask you to select the drive you can now select your 2nd drive on the machine, notice that even with this option enable it will not allow you to select your primary drive, and finally write the disk.
Make sure that once its done you plug it in to the machine you want to use for Batocera only Bootable OS and it will install flawlessly booting directly to Batocera every time you turn it on. (Also make sure that your BIOS boot setting most be set to legacy ) Etcher would create an small partition on the HDD to install the OS and on the first boot, Batocera will create a 2nd linux partition of the HDD remaining storage and auto assign it as the internal storage, keep in mind that you can only put rom here directly from Batocera file explorer, via SSH or plugin the HDD to a rooted linux pc.
This should work great for you until nadenislamarre creates a self install version of the OS, but to tell you the truth, I prefer it this way because if I want, I can just swap the HDD to another machine and it works like a champ.
By the way nadenislamarre, excellent work on the Software, I been very please and you have actually make one of my biggest wish come truth when it come to gaming, been able to use and actually PC to run all of my retro system in excellent performance its just amazing, thank you very much, now the only thing I have not been able to do its knowing how to load the 2nd disc on the game the require multiple disc to complete on consoles like ps1, or dreamcast, if would be nice if you could help me with that, I already made a separate post with the request, thanks again.