Is it possible to find unique (NON-duplicated) files?

What I would like is the ability to find files in directory tree 'a' that do not also exist in directory tree 'b', regardless of the name or location of the file.

Use case: I have a massive tree with a bunch of personal photographs in it. Over time, many of these files have been copied to other places on my drive so that I could do bulk operations on some subset of them -- zip some up for FTP, create a web album, email them, etc. Now I have duplicats all over my drive.

The duplicate file finder will work but it is a lot of effort. Lets say I have a folder named 'Pictures for emailing to X'. I'm pretty sure all of the files in there are duplicates and the whole folder can be deleted. But I would like to know for sure. It would be extremely helpful if Directory Opus just listed the files in 'Pictures for emailing to X' that do not also exist in 'All Pictures'. If the list is empty I can just delete 'Pictures for emailing to X'. That saves me hundreds of individual manual deletes in the duplicate file finder.

Why do you have to do hundreds of manual deletes rather than using the Duplicate Finder's Delete Mode to automatically select and (if you then want to) delete all but one of the duplicates?

Good question. I confess that my ignorance of the workings of delete mode prevented me from checking that box. My concern is that it will delete the wrong files. I will read up on it more.

If you run the Duplicate Finder in Delete Mode then it will show you which files it will delete. It won't delete any until you click the Delete button that's next to the Find button.

It will select all-but-one of each group of duplicates for deletion.

It selects them in the order they are displayed. That means you can change the sort order to change which files will be deleted/kept. (e.g. Sort by Location, or reverse-sort by Location, if you want to select everything in Folder A and keep everything in Folder B.)

There's also a Select button, in between Find and Delete, which lets you change the sort order and apply a new selection without wasting time doing the whole duplicate find again.