How to get there
This alert appears if you choose Move/Copy Layers from the Edit Menu, then set up the options so that you'd be copying over music that already exists.
Tip: If you do remove music that you wanted to keep, choose Undo from the Edit Menu.
What it does
Click the Selection Tool, and select a region of measures. Choose Move/Copy Layers from the Utilities Menu. In the dialog box, specify that one layer’s contents be placed into a layer that already contains notes. Click OK.
See Also:
