The internal usb and SD card slot are just there in case you want to boot from usb or SD.
You can install your OS to them and then all of your HDDs can be used for storage instead of losing capacity to an OS partition.
Back in the day, we would look for the newest SmartStart CD to install from, but it’s no longer possible to get from official channels. Also they’re only useful for installing the supported operating systems.
The G7 knows the thermal profile of most HP hardware of it’s generation and runs the fans at the appropriate speed for proper cooling for known hardware, but sticking anything 3rd party (or just newer) in there will ramp it up to max cooling mode for thermal protection. It’s a pain.
I found an answer on StackExchange that refers to a tool called rsync-sidekick which looks like it could achieve your aims.
Edit: There is also another answer on that same StackExchange page referring to a script called rsync-prepare which is capable of working with a remote destination.