Problem: CAD data spatial references are lost
ArcCatalog loses all spatial reference information for CAD data after properties are specified.
CAD spatial reference information defined in ArcCatalog must be saved to a projection file (.prj).
Solution or Workaround
Click Save As in the Spatial Reference Properties dialog box and save the definition as a .prj file.
To define a spatial reference for CAD data, see the ArcGIS Desktop Help topic within Contents > ArcCatalog > Working with CAD data > "Defining a CAD layer’s coordinate system."