Both the replies that u have recd earlier are valid but there is a subtle difference between the above techniques.
When you use Ext Copy Geom, you will need to specify the location for the surface to be copied (the best location would be a common co-ord sys) where as in the copy geom method, since the part used for copy and the part to be copied into are in the same assembly, you do not have to specify location.
Hope this helps to you decide which method to use and when.