editing in Datagrid View in vb.net

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VB

DataGrid

I have populated a datagridview using a sqldatareader / datatable which is having 3 columns.
Further I want to edit 3rd column in datagridiew. Though it is allowing editing in 1st column but not in 2nd & 3rd column. Plz help. Here is snippet of my code.

Private Sub cmdGetName_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles cmdGetName.Click
    Dim sqlcmd As New SqlCommand, reader As SqlDataReader, dTable As New DataTable
    sqlcmd.Connection = cnRM
    sqlcmd.CommandText = "select * from RMPARAS"
    cnRM.Open()
    reader = sqlcmd.ExecuteReader
    If reader.HasRows Then
        dTable.Load(reader)
        dgv.DataSource = dTable
        For Each column In dgv.Columns
            column.SortMode = DataGridViewColumnSortMode.NotSortable
        Next
        dgv.ReadOnly = False
        dgv.Refresh()
        cnRM.Close()
    End If
End Sub

Private Sub dgv_CellClick(ByVal sender As Object, ByVal e As System.Windows.Forms.DataGridViewCellEventArgs) Handles dgv.CellClick
    dgv.CurrentCell.ReadOnly = False
    dgv.BeginEdit(False)
End Sub

Exception thrown is

Error happened : Context : Commit : Exception : System.Data.ReadOnlyException: Column 'VALUE' is read only.
at System.Data.DataRow.set_Item(DataColumn column, Object value)
at System.Data.DataRowView.SetColumnValue(DataColumn column, Object value)
at System.Data.DataColumnPropertyDescriptor.SetValue(Object component, Object value)
at System.Windows.Forms.DataGridView.DataGridViewDataConnection.PushValue(Int32 boundColumnIndex, Int32 columnIndex, Int32 rowIndex, Object value)