Took me a while to find this out. Finally I found this threat that describes the procedure:
Seems you have to write a vanilla image to your sd in order to get it bootable. After that you can delete all files and put your own backup on it. Important point is to then change the current uuid in boot/boot.ini (although it says you shouldnt). That worked for me.
In many posts it is proposed to take a backup of the first 4 MB from your sd by dd. But restoring it is only possible on exactly the same type of sd, which will typically not be the case if you want to make a clone. I think this is because the first 4 MB do not only cover the u-boot, but also the partition table. So the partition/file system may be corrupted after writing the first 4 MB on a new sd. You have to recreate the partition table afterwards. But the system will not boot. Even when putting the right UUID in boot/boot.ini. Maybe fdisk is somehow corrupting the u-boot when writing the partition table.
So I see no point in backing up the first block of the sd. It will help you only if you want to use exactly the same sd again.
I had the same problem on first boot (buster on HC2). Second boot worked. Didnt find out the reason for failure on first boot (suspectzed an error in ssh config, but that wasnt the case).
