Another Idea I had, was mounting an external HDD over USB according this solution. Unfortunately, this is above my pay grade…
Edit: I gave it a try but got stuck with “os datadisk move /dev/xxx” I get the message “Error: /dev/sdc is not a harddisk”. It’s a USB 3.0 SATA HDD, fdisk shows “Diskmodel: Esternal USB 3.0”. I changed from DOS to GPT, but no change.
Could anybody help?