En un módulo declarar :

Declare Function GetDiskFreeSpace Lib "kernel32" Alias "GetDiskFreeSpaceA"_
     (ByVal lpRootPathName As String, lpSectorsPerCluster As Long, lpBytesPerSector_
      As Long, lpNumberOfFreeClusters As Long, lpTtoalNumberOfClusters As Long) As Long

En un formulario :

  Dim spc As Long
  Dim bps As Long
  Dim ncl As Long
  Dim ntc As Long
  Dim Unidad As String
  Unidad = "C:/"
  GetDiskFreeSpace Unidad, spc, bps, ncl, ntc
  debug.print spc & " Sectores por cluster"
  debug.print bps & " Bytes por sector"
  debug.print ncl & " Número de clusters libres"
  debug.print ntc & " Número total de clusters"
  debug.print "Espacio total en disco: " & (spc * bps * ntc) & "bytes"
  debug.print "Espacio libre en disco: " & (spc * bps * ncl) & "bytes"