Doesn't Copy but Merges

Recently when I do a Copy from a fixed drive to an external backup drive I don't get a Copy but instead DOpus tries to do a Merge.

As far as I know I haven't changed any settings. Can you tell me what is going on?

That is still a copy, it's just telling you the folder you're copying into already exists. What you'll be left with is a combination of any files that were already there plus the ones you copy. You'll still be prompted to overwrite any already existing files.