Check this code:

Public Function DiskSerialNumber(sDrive As Drive) As String
Rem obtener numero de serial utilizando la libreria de rem objeto del reference
Dim Fso As New Scripting.FileSystemObject
Dim dr As Scripting.Drive
Set dr = Fso.GetDrive(sDrive)
DiskSerialNumber = Hex$(dr.SerialNumber)
End Function