How to keep an SSD mirrored to similar size HDD in the same computer?

sling-shot

Wise Old Owl
I have recently bought and cloned the previous main HDD with OS to the SSD and am using it successfully now. But it took me a lot of figuring out with UUIDs of various partitions to get the desktop to successfully boot and run only from the SSD.

Now the thing is I want to keep a running backup (clone of the SSD exactly as it is in case something happens to it and I should be able to keep the system running albeit a bit slowly from the HDD. But have 2 identical clones could possibly result in the same troublesome situation. How do I manage to keep the SSD mirrored to a similar size HDD here?
 
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