Identifying files which are not duplicated

My Macbook Air is running out of space with documents of various types on different topics littered throughout the internal drive

I am currently organizing these by copying them, folder by folder, to an external drive where they can be reorganised and duplicates weeded. When this is done I’ll delete the originals from the Mac’s internal drive. But when I get to that stage I’ll need to be sure that they all exist somewhere on the external drive.

I know there are various means of identifying duplicates, but what I need is a way to identify non-duplicates remaining on the internal drive so I can decide what, if anything, to do with them.

So I think I need a two-part process that will:
- compare the internal and external drives and apply the tag ‘DUPLICATED’ to the internal files that also exist on the external, and then,
- re-examine the internal files and where they are not tagged ‘DUPLICATED’ either tag them ’UNIQUE’ or list them. Perhaps a listing is better, to avoid tagging system files which might not be a good idea.

Is this something I can do with Hazel Rules?

Thanks.

You might be better off using a specific duplicate finder app as that is more interactive. I don't have any specific recommendations but there are a few out there.