If you are dragging a file on the same harddrive. Then The default behavior is Move. if source and destination drives are different then it is Copy.
You can toggle the method by holding the SHIFT key. (The mouse cursor will change. a
You can also drag and drop using the right mouse button. You then get a popup menu when you drop the file where you can select copy or move.
(If you are running CommanderStyled settings, you need to active this in the Explorer Panel Settings under mouse configuration)