Not sure if it's helpful, but I was having this same problem, system wouldn't boot, error screen mentioned:
mount: /sys/firmware/efi/efivars: unknown filesystem type 'efivarfs'.
Since I couldn't boot and Unraid runs bare-metal on my server, I decided to just get my flash drive and downgrade it in my own PC, but before I tried to just change the USB Port the flash drive was on and... it worked. V7.0.