I have two external content types, both from the same external source. Now I want to create search scopes for each of the external content types.

My first try was to filter by content type. Unfortunately this is not possible as the managed metadata property has the value for Retrievable set to false. Timmy Gilissen explains how to see which properties can’t be retrieved: http://www.moss2007.be/blogs/timmy/archive/2011/08/10/core-results-webpart-fetched-properties-issue.aspx

The solutions that I have found is certainly not ideal, but it works in my case:
Since each external content type hase a seperate web part page bound to the as default page, I was able to define the search scopes, by using the existing managed property ‘FileName’.