Public Class Form1
  Private selectedIndex As Integer = -1  
 
  Private Sub Form1_Load (...) ...
    Me.cbDrives.SelectedIndex = 0
  End Sub
 
 
  Private Sub cbDrives_SelectedIndexChanged (...) ...
    If Me.cbDrives.SelectedItem.ToString = "xyz"
      Beep
      Me.cbDrives.DroppedDown = True ' ??? soll offen bleiben ???
    Else
      Me.selectedIndex = Me.cbDrives.SelectedIndex
    End If
  End Sub
End Class