I've been puzzled why unduplicated content on a drive with SMART warnings and damage was not being removed, despite setting various balancers to do so.
I discovered today part of the reason - possibly all of it. I believe all the remaining unduplicated content were backup images from Macrium Reflect. Reflect has an anti-ransomware function that can prevent any changes to backup files. This was preventing drivepool from removing them. I realized this when I shut down drivepool service and tried to manually move those files to another drive.
I'd have expected drivepool to report that files were inaccessible - but apparently it did not know other software was actually blocking it - which brings me to the next issue.
Stablebit Scanner reported 20 unreadable sectors on this drive and 3 damaged files. SMART had indicated 5 pending sectors. I decided to re-run the scan after disabling the Macrium Image Guard - so far, it appears the unreadable sectors may have been caused by Image Guard and may not be bad.
Remains to be seen whether the 5 pending sectors will end up being reallocated or become readable.
The damaged files however were NOT image backups, so it's unclear if there was any connection.
Bottom line: don't use Macrium Image Guard (or any other similar software) with pooled files. I may just move my image files out of the pool to avoid the issue.