Thanks artie. I had found (attempted to try) those things but was getting nowhere, which is why I thought it might be different with El Capitan. However, this morning both work like blazes. Perhaps the gods of things digital are having a laugh at my expense.
iMac (Retina 5K, 27", 2017), 3.4 GHz Intel Core i5, 8GB RAM, 2400 MHz DDR4
OS Mojave 10.14.6
Canon MX710 Printer
Epson Perfection V500 Photo Scanner
Time Machine on 1TB LaCie USB-C
Carbon Copy Clone on 500GB OWC Mercury OTG Pro