I’ve added the following cron job:
0 * * * * cp -ru /home/purism/Pictures/* /media/purism/"Memory card"/Pictures/
This copies, every hour on the hour, the contents of the Pictures directory (internal storage) to a directory I’ve named “Pictures” on the memory card, avoiding creating duplicates.
Instead of moving the files, which would remove them from the internal storage, this way leaves me with the original and a copy, just in case. I can adjust the interval to something more frequent if I’m shooting a lot of photos at any given time, during a trip, for instance.
The .jpg files seem to size between 1 and 2 megabytes, and the accompanying .dng files at about 13 megabytes, so they don’t take up a lot of space. I can just do an occasional deletion of accumulated photos in internal storage after my backups are created.
EDIT: “Memory card” is how my microsd card displays in the file structure. A different card might show a different label.