Hi Peter,
Does the external project have access to index files? SearchIndexInfo is a DB based object that contains configuration for the index, but actual index binaries which are used to search are in App_Data\CMSModules\SmartSearch{IndexName} folder. These have to accessible. For MVC applications, which are external projects as well, we are actually using web farms to move indexes in application memory.
In your situation, you can check whether your application can actually access above mentioned folder on main server and that the path is mapped correctly for this server.