Exporting 4.5 million records to text file from temp datatable

I have 4.5 million records in a temporary datatable, my goal is to export data to a text file in chunks of records until all records are written to textfile or processing

Can someone help me write the logic to export 750,000 records at a time until all records are gone. For every 750,000 records create its own text file. Below is the logic I using below in code, can you guys help me modify logic below


DataTable  dataTable = new DataTable ();

dataTable.Columns.Add(new DataColumn("UNIT", typeof(string)));
                        dataTable.Columns.Add(new DataColumn("SERIAL", typeof(string)));
                        dataTable.Columns.Add(new DataColumn("PART", typeof(string)));

SqlDataReader dr = cm.ExecuteReader();

                        while(dr.Read())
                        {

                                nextRow = dataTable.NewRow();

                                nextRow["UNIT"] = row.UNIT;
                                nextRow["SERIAL"] = row.SERIAL;
                                nextRow["PART"] = row.PART;

                                dataTable.Rows.Add(nextRow);
                           
                        }