Ahhh. So simple! Thanks!

Drag & Drop to add files to the Library doesn't move them, it just scans them into the Library. You may have had Tools > Options > Auto-Organize enabled, it will automatically move files.

Actually, do you have folder monitoring running (did you in the past)? If not, enable first and try again if you already tried TEST.

Do you remember from past if what you were dragging and dropping had both source and destination folders in library.

A quick way to test is to create a TEST folder, so named, on the external drive.
Copy a file to it in explorer (with MM running) > scan the TEST folder into library > drag and drop as you claimed to destination (TEST), see what happens

No, it@s on an external drive. Version is 4.1.16.1386

Is the source folder, that you're dragging from, also in the library?
What version of MM (numbers)?

All of my library files are in one folder on my PC. It used to be if I dragged files from a folder outside of this into 'Music' in MMW's left side menu tree it would automatically move them from that folder into the one all my other files are stored in. This is not the case now and I'm having to physically move them.