I wonder if you’ve had the situation that I had before where you have to attach a database with one or few missing .ndf files. It seems pretty common since I’ve just seen the same scenario at one of my clients. The reasons behind it are quite similar – They created a new file group with files to host some temporary data for data fixing or testing. New files were not created under the folder as usual and they thought they would remove them right after tasks complete. But the latter part got forgotten afterwards.