Hi @n3o, are you using Windows? Linux ? OpenBSD? binary or compiled rkflashtool?
This is because
* on Linux maybe you don't have the right USB permission. You can switch to the super user account (super easy), or write an udev rule that chmod the USB node(easy if you know what are you doing)
* my rkflashtool uses libusb which on Windows needs the generic USB driver called WinUsb. With Windows 7 you need to right-click and install this file https://raw.githubusercontent.com/ilmich/rkflashtool/master/rockwinusb.inf. With Windows > 7 there is no need to install a driver, but you need to associate the unknown device with the USB generic driver WinUsb (I'm not a Windows user so try starting reading this link https://github.com/libusb/libusb/wiki/Windows#user-content-How_to_use_libusb_on_Windows)
* there is a bug on Windows (fixed with the latest alpha version) with multiple USB devices connected. Try, if you can, to have only the box connected with USB.
About your experiments, you wrote a lot, but I'm sorry, I'm too lazy to read.
So if I can give you some advice, format the internal storage and forget it. Write the Armbian version you like on the SD card and stop.
This is because in addition to having a few problems, sd cards with 64/128GB are cheap and for Linux use are better than the 'little' 8 GB of internal storage that you often find in our boxes.
Cheers
Michele
edit: rkflashtool is an external tool made with love by volunteers. Obviously, there is also the contribution of @jock and @fabiobassa, but for any problem with the tool, I ask you the courtesy of opening an issue here https://github.com/ilmich/rkflashtool/issues