Vs2010 Data Generation Plan fails with "Data generation failed because of the following exception: Column "xyz" does not allow DBNull.Value"
So apparently according to this thread the data generation plan blows up when you have a table containing only a primary key and no other columns. It turns out that one of my independent entities, whose only purpose is to serve as a joinder to one of my other tables, fit this description. After adding a harmless Description column, I was able to proceed fixing other problems until the generation plan completed successfully.