One week later:
With the computer in the Sleepless mode described above, there have been no improper disk ejects.
I can only guess that there is some issue with external drives and sleeping. Didn't seem to matter if the connection was Firewire or USB; the disk would get "lost" either way.
On a Mac since 1984.
Currently: 24" M1 iMac, M2 Pro Mac mini with 27" BenQ monitor, M2 Macbook Air, MacOS 14.x; iPhones, iPods (yes, still) and iPads.
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