C#

foreach(DataGridViewRow row in dgv1.Rows)
{
   dgv2.Rows.Add(row);
}

No row can be added to a DataGridView control that does not have columns. Columns must be added first

foreach (DataGridViewColumn c1 in dataGridView1.Columns)
            {
                DataGridViewColumn c2 = (DataGridViewColumn)c1.Clone();

                dataGridView2.Columns.Add(c2);
            }

Row provided already belongs to a DataGridView control.

foreach (DataGridViewRow row in dgv1.Rows)
{
   object[] cells = new object[row.Cells.Count];
   row.Cells.CopyTo(cells, 0);
   dgv2.Rows.Add(cells);
}