Your suggestion got me past my mystery -- thank you.
I was starting to suspect that Mojave, or my Mac Pro (Late 2013) and Mac Mini (Late 2014), had a shortcoming with USB. Success with your suggestion corroborates my suspicion regarding Mojave.
I had been using an OWC Mercury Elite-AL Pro since the Snow Leopard days, using FireWire 800, without issue all the way up to Mojave. It still runs fine, although it is rather slow and noisy by current standards. It's replacement is an OWC Mercury Elite Pro using USB 3, formatted with APFS and partitioned with HFS+. I opted for HFS+ because I read that HFS+ performs better with disk drives, as opposed to SS drives.