Samsung Chromebook Pro CAROLINE Board Flashing Issue

Recently I’ve been trying to get Linux to run on my Samsung Chromebook Pro (Caroline board) but when I run the firmware utility script from Mr Chromebox and try to save a backup of ChromeOS to the microSD I have inserted it fails to detect any connected USB/SD devices to save the backup to. I have to use microSD because the only other ports are for headphone jacks and USB-C connections and I do not have a USB to USB-C adapter. How can I get the script to detect my microSD and save the backup to it? I would very much like to have a backup as I don’t want to be stuck if flashing the firmware goes wrong somehow.

I am also thinking of just buying a USB-C drive. If I go that route, how many gigs would I need to store the ChromeOS image?

the firmware backup is 16MB for CAROLINE.

you can always make a backup manually and copy to the SD card. Assuming you are logged in to ChromeOS (guest ok)

sudo flashrom -p internal -r ~/Downloads/stock-firmware-CAROLINE.rom

then use the ChromeOS file manager to copy from the Downloads folder to SD card