TheDiamondCG It works exactly the same as installing on USB. The easiest way is to take the HDD out and put it in your current PC. Then use (for example) etcher in unsafe mode so it detects your HDD and put the image on. Put the HDD in the old PC and fire it up -> voila Batocera ;-) I have two HDDs in my old PC one 80gb with the system and another 1TB with roms. As the PC is connected on the same network as my main PC I copy all I need via network. You can check your IP in system settings -> information and then just connect to it via shortcut i.e. 192.168.0.10. Depanding on your PC setup, especially GPU you might want to consider the 5.22 version with legacy driver. I have older nvidia so 5.22(legacy) actually runs some newer systems better than 5.23.
batocera.linux 5.22 with nvidia-legacy drivers (390.xx) (poke)
batocera-5.22-x86_64-20190608.img.gz (download for x64 systems):
Also Linux reader(https://www.diskinternals.com/linux-reader/) is a good program to view Linux files under windows if you choose to re-connect the HDD to your PC to copy files. Just remember to actually run it first after putting it on the disk on your old PC. If you use two HDDs make sure you select the second HDD as the one to use for roms inside of Batocera. And enable nvidia in your system via putty(https://www.putty.org/) for example as exaplained in poke post:
mount -o remount,rw /boot
batocera-settings /boot/batocera-boot.conf --command uncomment --key nvidia-driver
mount -o remount,ro /boot
You can do the same by pressing F1 on your main system screen inside of Batocera and going 'up' then you will find 'boot' folders and re move the '#' from before "nvidia-driver=true" inside "batocera-boot.conf".
Some people also use USB for system and the internal HDD for roms. This gives you couple of giga for games on the Disk and easy access to USB to re-install if something goes wrong ;-)
I hope it all made sense and enjoy ;-)