I know about Clonezilla and copy pasting partitions with gparted, but can I just use dd to copy a partition with batocera to a USB stick and will it then boot from the stick? Do I have to set the boot flag or take any other steps?
Thank you for any tips.
One partition is not enough. You also need to copy a EFI boot partition and create a correct partition table manually. And note that you cannot get a correct result when partitions you are copying are mounted. You need to boot some live system to do this.