mokanman

You just need to understand where the bootloader on ARM boards lives and how to transfer this to an image or another SD card. Usually all this stuff lives inside the 1st MB of the SD card so cloning the first MB with dd the first time should just work. But if then later u-boot updates are provided via our Armbian repo you won't get updates on your backup image.
 
Most probably the best idea is to merge the functionality present in our nand-sata-install tool with your backup script (since nand-sata-install has to take care about writing this bootloader stuff you can study there how it works -- it's the /usr/lib/u-boot/platform_install.sh include)

Available on Amazon and from most SBC companies, most are 3.3 volt logic, some (Odroid XU4) are 1.8 volt.  It is a USB to serial uart adapter.

Great minds think alike!  That's exactly what I've been working on for a couple of hours now.  When I first tried to copy, it told me that there wasn't enough space.  It told me that the 277.6 GB (on my 16 GB card!) was too much to copy to my other 16 GB card.  I drilled down to a find a file in /lib/udev/rules.d called "90-alsa-restore.rules" that was showing a supposed size of 274.9 so I copied all the files but that one (it already existed there with same modified date anyway but only 489 bytes!).  File copy is taking a long time.  If this doesn't work then I'll just start from scratch I guess.