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I am using the following script to backup my root partition to sd (it is just a slightly modified Armbian script - please adapt the device name if necessary):
 
# cat backuptosd.sh #!/bin/bash # Check if user is root if [ $(id -u) != "0" ]; then echo "Error: You must be root to run this script." exit 1 fi cat > install-exclude <<EOF /dev/* /proc/* /sys/* /media/data1/* /media/data2/* /media/data3/* /media/data4/* /media/data5/* /mnt/sd/* /mnt/ssd/* /mnt/usb/* /mnt/hd/* /run/* # /tmp/* # /root/* EOF exec 2>/dev/null umount /mnt/sd exec 2>&1 mount /dev/mmcblk1p1 /mnt/sd rsync -avxSE --delete --exclude-from="install-exclude" / /mnt/sd # change fstab sed -e 's/UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx3c/UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx2d/g' -i /mnt/sd/etc/fstab sed -e 's/UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx3c/UUID=xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxx2d/g' -i /mnt/sd/boot/armbianEnv.txt umount /mnt/sd rm install-exclude  
The UUIDs need to be inserted (blkid is your friend) - the leftmost is the one of your root system, the other one is the UUID of the sd in this example.
If you need a bootable system on sd - the easiest way would be to start with a fresh Armbian image flashed to the sd card and to boot from it at least once in order to expand the filesystem.
Then you may boot from your main root partition and simply sync it to the sd card using the above script.
 

In case of disk failure... just copying the filesystem to a backup, which then is copied to a new disk, which is inserted to slot A and booted. It will not boot. I presume so, by the fact that I tried to repartition/resize that disk holding system in slot A and it failed to boot after that. So, there is something more involved, hints are visible by looking at mounts. I would like to get the system backupped. Know the needed bits and tweaks, so that I know that I can do it and it will work, in case of disk failure.

@SIGSEGV I did compile zfs 2.0.0 and its up and running.  Rsynced 1.4TB to Helios box after that. Seems to be working. Next, home assistant, after that's up and running I'll get rid of my ATX computer that has been doing those jobs...

@SIGSEGV I did compile zfs 2.0.0 and its up and running.  Rsynced 1.4TB to Helios box after that. Seems to be working. Next, home assistant, after that's up and running I'll get rid of my ATX computer that has been doing those jobs...

I actually managed to get the zfs working, by following the post by  @grek in this very thread. I've been running some fio tests to see if everything is dandy. Seems to be. :-) 
root@helios64:/ironwolf # zpool --version
zfs-2.0.0-rc6
zfs-kmod-2.0.0-rc6
root@helios64:/ironwolf # zfs version
zfs-2.0.0-rc6
zfs-kmod-2.0.0-rc6
root@helios64:/ironwolf #
 
I've noticed that zfs 2.0.0 has been released, so that means that I'll be compiling it after the fio runs are done (they've been running for a day or two now...) .