Underlays that are no longer needed can be detached from a drawing file.
When you detach an underlay, all instances of the underlay are removed from the drawing, and the linking path to the file is removed.
To hide the display of an underlay temporarily, you can unload it rather than detach it. This action preserves the underlay location for reloading later.
Erasing 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.