Nov 3, 2018, 4:32 PM by dpchrist@holgerdanske.com:
dd(1) is a lowest-common-denominator tool that is available on most every Debian system, live CD, and installer (plus other Linuxes and BSD's). I use dd to blindly copy bytes to bytes, so it is unaware of partition tables, slices, partitions, volume managers, encryption, file systems, OS's, drivers, apps, data, etc... It's fast and I avoid the headaches. It should work well for your use-case.
I actually had a working backup system in place, rsync-ing "/" and just copying /boot files from source HD to backup HD. It worked reasonably well over the years and I now wonder how simply copying /boot files did not cause me problems back then. I think I probably did initial mirroring with dd and after that just did incremental updates with rsync. Did not even have issues when both HD were present in the system.