//声明:
GetDiskFreeSpaceEx(
  lpDirectoryName: PChar;                          {磁盘根路径}
  var lpFreeBytesAvailableToCaller: TLargeInteger; {可用空间}
  var lpTotalNumberOfBytes: TLargeInteger;         {总空间}
  TotalFree: PLargeInteger                         {剩余空间}
): BOOL;


//举例:
procedure TForm1.FormCreate(Sender: TObject);
var
  d1,d2,d3: Int64;
begin
  GetDiskFreeSpaceEx('C:',d1,d2,@d3);

  Memo1.Clear;
  with Memo1.Lines do
  begin
    Add(Format('可用空间: %f GB',[d1/1024/1024/1024]));
    Add(Format('总空间: %f GB',[d2/1024/1024/1024]));
    Add(Format('剩余空间: %f GB',[d3/1024/1024/1024]));
  end;
end;


//效果图: