最直接的方法是调用API函数 GetDiskFreeSpace,函数声明如下:
    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
    计算规则:磁盘上剩余空间(字节)=磁道(Sector) *剩余扇区 *每扇区的字节数