Yes I have fitted a https://www.amazon.co.uk/dp/B08V83JZH4?psc=1&ref=ppx_yo2ov_dt_b_product_details
a 980 m.2 1T drive.
install the latest bios from ASUS G20CB page with the lastest app to do so.
buy a heat sink for the m.2 drive to be careful.
installing is then possible.
make a windows 10 install drive on the fastest usb key you have,
install the latest bios.
run load optimised defaults.
install the new drive, back in the bios while not needed disable all the ssd drives in bios but not the chipset so disable each drive individually, this will make sure your installing windows onto the m.2 drive and making sure the mother board asides it to be the boot drive.
Insert the windows usb key you have made and restart your G20CB you may have to cycle the system to get the bios to see the new setup but if it’s all working you should boot to the usb and windows setup should start at the choose a drive screen you should see your m.2 drive you will only be able to tell by its size hence disabling all other drives to cut the chances of putting windows on the wrong drive.
you may get an error, about GPT file format if you do this can be fixed here with command prompt, shift f10 to open command prompt.
then just install on windows you may have to click refresh in the windows install dialog.
all should work, once your working you can re activate you ssd drives in bios.
the physical install of the m.2 drive is covered in other builds the how to take the G20cb apart if you buy the heat sink kit I got the little screw you need was included.
Heat sink for m.2 I used.
If your system is missing screws, and you need an m.2 fixing screw.
This cable fits the 8 pin slot for the graphics card power output on the mother board if your going ... There may be others but I can confirm it fits.