IEnumerable<DataRow> added = table1.AsEnumerable().Except(table2.AsEnumerable());
should be changed to
IEnumerable<DataRow> added = table1.AsEnumerable().Except(table2.AsEnumerable(),DataRowComparer.Default);
Because DataRows don't know how to compare themselves to eachother on their own. You can also provide your own equality delegate instead of DataRowComparer.Default if required.