'Para saber el espacio libre de un disco duro
Private Declare Function GetDiskFreeSpace Lib "kernel32" Alias "GetDiskFreeSpaceA" (ByVal lpRootPathName As String, lpSectorsPerCluster As Long, lpBytesPerSector As Long, lpNumberOfFreeClusters As Long, lpTotalNumberOfClusters As Long) As Long

'en cualquier parte del programa...

Dim strRootPathName As String
Dim lngSectorsPerCluster As Long
Dim lngBytesPerSector As Long
Dim lngNumberOfFreeClusters As Long
Dim lngTotalNumberOfClusters As Long

Dim strDrive As String
Dim strMessage As String
Dim lngTotalBytes As Long
Dim lngFreeBytes As Long

    strDrive = "C:\" 'drive letter
    
    If GetDiskFreeSpace(strDrive, lngSectorsPerCluster, lngBytesPerSector, lngNumberOfFreeClusters, lngTotalNumberOfClusters) = 0 Then
        strMessage = strMessage & vbCrLf & "An error occurred."
    Else
        strMessage = strMessage & vbCrLf & "Sectors Per Cluster: " & Format$(lngSectorsPerCluster)
        strMessage = strMessage & vbCrLf & "Bytes Per Sector: " & Format$(lngBytesPerSector)
        strMessage = strMessage & vbCrLf & "Free Clusters: " & Format$(lngNumberOfFreeClusters)
        strMessage = strMessage & vbCrLf & "Total Clusters: " & Format$(lngTotalNumberOfClusters)
        lngTotalBytes = lngTotalNumberOfClusters * lngSectorsPerCluster * lngBytesPerSector
        strMessage = strMessage & vbCrLf & "Total Bytes: " & Format$(lngTotalBytes)
        lngFreeBytes = lngNumberOfFreeClusters * lngSectorsPerCluster * lngBytesPerSector
        strMessage = strMessage & vbCrLf & "Bytes Free: " & Format$(lngFreeBytes)
        strMessage = strMessage & vbCrLf & "Percent Used: " & Format$(1 - (lngFreeBytes / lngTotalBytes), "0.00%")
    End If
    
    MsgBox (strMessage)