Option Compare Database
Option Explicit

Private Type LARGE_INTEGER
    lowpart As Long
    highpart As Long
End Type

Private Declare Function GetDiskFreeSpaceEx Lib "kernel32" Alias _
    "GetDiskFreeSpaceExA" (ByVal lpRootPathName As String, _
    lpFreeBytesAvailableToCaller As LARGE_INTEGER, lpTotalNumberOfBytes _
    As LARGE_INTEGER, lpTotalNumberOfFreeBytes As LARGE_INTEGER) As Long

Sub Test()
    Dim lResult As Long
    Dim liAvailable As LARGE_INTEGER
    Dim liTotal As LARGE_INTEGER
    Dim liFree As LARGE_INTEGER
    Dim dblAvailable As Double
    Dim dblTotal As Double
    Dim dblFree As Double

    'Determine the Available Space, Total Size and Free Space of a drive
    lResult = GetDiskFreeSpaceEx("E:\", liAvailable, liTotal, liFree)

    'Convert the return values from LARGE_INTEGER to doubles
    dblAvailable = CLargeInt(liAvailable.lowpart, liAvailable.highpart)
    dblTotal = CLargeInt(liTotal.lowpart, liTotal.highpart)
    dblFree = CLargeInt(liFree.lowpart, liFree.highpart)

    'Display the results
    Debug.Print "Available Space:  " & dblAvailable & " bytes (" & _
                Format(dblAvailable / 1024 ^ 3, "0.00") & " G) " & vbCr & _
                "Total Space:      " & dblTotal & " bytes (" & _
                Format(dblTotal / 1024 ^ 3, "0.00") & " G) " & vbCr & _
                "Free Space:       " & dblFree & " bytes (" & _
                Format(dblFree / 1024 ^ 3, "0.00") & " G) "

End Sub

Private Function CLargeInt(Lo As Long, Hi As Long) As Double

    'This function converts the LARGE_INTEGER data type to a double

    Dim dblLo As Double, dblHi As Double

    If Lo < 0 Then
        dblLo = 2 ^ 32 + Lo
    Else
        dblLo = Lo
    End If

    If Hi < 0 Then
        dblHi = 2 ^ 32 + Hi
    Else
        dblHi = Hi
    End If

    CLargeInt = dblLo + dblHi * 2 ^ 32

End Function