In the External References palette, in
the File References pane, select the underlay you want to detach.
Right-click on the underlay and click
Detach.
The underlay is no longer
linked to the drawing file. All instances of the underlay are removed
from the drawing.
NoteErasing an individual
instance of an underlay is not the same as detaching it. An underlay
must be detached to remove the link from your drawing to the file.