Bug 1: If i choose "Select only Folder X", then i ALSO get all the subfolders UNDER folder X. That should not happen.
Bug 2: If i select D: drive entire SD card, the scan runs all night, many hours, never ends.
Bug 3: When i add one or more folders (including ALL folders on the drive in "Bug 2" above) and search, i get "File search completed" immediately, and no results.
For a drive specified 'All files' as you have there (D:\*.*) then it will search subfolders of that drive.
If you had specified a filetype (D:\*.txt) you then have the option to search subfolders or not.
I note that you have 'Content' selected for the matching - that can take a long time depending on what type of files you are searching, and how big they are.
(Which is why there is an 'ignore files size over' option).
TBH though it may just be something specific to the filesystem the SD card is formated to?
I don't know about #3 without seeing the path(s) specified, in particular the wildcard characters.
Quite often when a duplicate search concludes immediately it's because the wildcards are not quite correctly specified.
Again though if that is with the SD then it may be related to the cards filesystem?
You need to have wildcard characters at the end of the searchpath you specify, eg. C:\Users\Name\Documents\*.* for all files or C:\Users\Name\Documents\*.txt for just txt documents, and so on.
If you look at both screenshots above you will see that 'System Files' is ticked under 'Ignore'.
That is the default setting and so unless a user unticks it System Files will be ignored.
(Advanced users may want to see the system files in special cases, an advanced user is unlikely to delete anything by mistake).
Duplicate Finder only finds duplicates, it doesn't delete anything unless you specifically select something that has been found and then tell it to delete it.
The rule is the same with any file, if you don't know exactly what it is then you should leave it alone.
Note that also for a file to be flagged up as a duplicate, it must exactly match ALL of the selected 'Match By' rules as well as not match ANY of the 'Ignore' rules.
As such, you may want to remove some of the Match By rules - for instance, only searching for same file name, or only searching by content (and size, as that's required by the content rule), that sort of thing.
(And when I say exactly, I do mean exactly - "Filename" and "Filename1" will not match by Name, files that are 1000kb and 1001kb will not match by Size, and if so much as a single byte is off in the Content, it shouldn't match, etc. It also has no knowledge of what the data "means". For instance, two photos that are at different sizes won't match via Size and Content as it has no way of analyzing it as a photo, just a very very long number.)