So far so good, after using "Reset Settings". I had to reboot a couple of times afterward to get rid of some glitches (e.g., Drivepool reported that its Service had not started, although the Services applet showed that it was running), but now things are looking like they are back on track.
See the attached screen shot.
There is no longer a duplicate of a Pooled drive listed under Non-Pooled.
Yes, I do have StableBit Scanner installed. It is still making its way though the process of scanning all the drives. It has now completed scanning drive K and reports some problems, which perhaps explains the zeroed targets that appeared for the drive in DrivePool before Scanner had finished scanning it. Scanner has not scanned drive L yet, but I expect that when it does, it will concur with Harddisk Sentinel, and zero that drive's targets as well.
The Measuring process for this pool completed without error. Balancing is proceeding without error so far.
I assume that Duplication of the remaining Unduplicated files did not occur before Balancing because the balancer that wants to clear the files off of drive K has priority. I assume that, unless I override that balancer, I will have to wait until K is cleared before I will see whether I am still getting the Duplication Inconsistent error. Since all files on K are duplicated, it looks like it is safe for me to defer the clearing of that drive.
My priority for now should be to Duplicate the remaining Unduplicated files on L, since HD Sentinel has reported that it is weak.
So, for now, unless you have different advice on how I should proceed, I will turn off the StableBit Scanner balancer, and will see whether that forces the Duplication process to start.
In any case, once I can see whether or not the Duplication process is working and no longer reporting the Duplication Inconsistent error, I will report back here with an update.
Thank you for your help.
David