Aca les dejo un snippet que muestra como obtener el espacio libre, total y disponible de una unidad y formatearlo.

Option Explicit
 
Private Declare Function SHGetDiskFreeSpace Lib "shell32" Alias "SHGetDiskFreeSpaceA" (ByVal pszVolume As String, pqwFreeCaller As Currency, pqwTot As Currency, pqwFree As Currency) As Long
Private Declare Function StrFormatByteSize64A Lib "SHLWAPI.DLL" (ByVal qdw As Currency, ByVal pszBuf As String, ByVal uiBufSize As Long) As Long
Private Declare Function lstrlen Lib "kernel32" Alias "lstrlenA" (ByVal lpString As String) As Long
 
Private Sub Form_Load()
    Dim cFreeCaller As Currency
    Dim cTotal      As Currency
    Dim cFreeTotal  As Currency
    Dim cFormat     As Currency
    Dim sBuff       As String
 
    If Not SHGetDiskFreeSpace( _
       Environ$("HOMEDRIVE"), _
       cFreeCaller, _
       cTotal, _
       cFreeTotal) = 0 Then
 
        Debug.Print "Drive: " & Environ$("HOMEDRIVE")
        cFormat = cFreeTotal:   GoSub Format: Debug.Print "Total Free Space: " & sBuff
        cFormat = cFreeCaller:  GoSub Format: Debug.Print "Free Caller: " & sBuff
        cFormat = cTotal:       GoSub Format: Debug.Print "Total Space: " & sBuff
 
    End If
 
    Exit Sub
Format:
    sBuff = Space(32)
    StrFormatByteSize64A cFormat, sBuff, 32
    sBuff = Left(sBuff, lstrlen(sBuff))
    Return
End Sub