Ah! That’s most likely because /media/storage/RX100m7 directory doesn’t exist. You might need to create it manually using the following commands:
sudo mkdir -p /media/storage/RX100m7
sudo chown -R pi:users /media/storage/RX100m7
sudo chmod -R 775 /media/storage/RX100m7
sudo setfacl -Rdm g:pi:rw /media/storage/RX100m7
I’ll see if I can add this to the backup script.