I probably would upgrade to Mojave, except my Mid-2011 iMac will not install it. High Sierra is the end of the road for this machine, and I have no desire to perform any hacks to get Mojave to run.
It's also possible this anomaly would transfer to Mojave anyway, since I forgot to mention I'd asked about this problem on another forum, and the fellow who responded also could not reproduce the issue.
So, it's something particular to my system or some odd bug in the way the Finder deals with attached clone volumes. Were any of the volumes you tried to move around clones of your boot drive? If so, were they cloned with SuperDuper?
I will write to the developer of SuperDuper and see if he's experienced anything similar.
