If you hold SHIFT while dragging your photos they will always be moved. If you hold down CTRL while dragging they will always be copied. If you drag using the right mouse button then you get a menu when you drop the item where you can choose the action to take. When dragging a photo from one drive to another the default action is to copy; when dragging between locations on the same drive the default is to move. The default action varies with other file types such as program files where dragging can create a shortcut.