Código (vbnet) [Seleccionar] 
Imports System.IO
Public Class Form1
    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        For Each vUnidad As String In Directory.GetLogicalDrives()
            ComboBox1.Items.Add(vUnidad)
        Next vUnidad
    End Sub
End Class