U盤容量的獲取

用DeviceIoControl得到U盤物理參數!
bResult = DeviceIoControl(hDevice,  // device to be queried
      IOCTL_DISK_GET_DRIVE_GEOMETRY,  // operation to perform
                             NULL, 0, // no input buffer
                            pdg, sizeof(*pdg),     // output buffer
                            &junk,                 // # bytes returned
                            (LPOVERLAPPED) NULL);  // synchronous I/O
總存儲字節為

totalSize=pdg.Cylinders.QuadPart * (ULONG)pdg.TracksPerCylinder *(ULONG)pdg.SectorsPerTrack * (ULONG)pdg.BytesPerSector;
如果是電子元件做的U盤實際存儲的字節數大于totalSize,用本本硬盤改的U盤就沒問題,請教這是什么原因!