关于磁盘大小的问题

要获取磁盘大小
我用如下两种方式获取的值为什么不一样啊?
1.. DeviceIoControl
2..GetDiskFreeSpaceEx
代码如下:
#ifdef  DISK_GET_GEOMETRY
HANDLE hDevice;              
BOOL bResult;                
DWORD junk;    
DISK_GEOMETRY dgt;              
#endif  // #ifdef  DISK_GET_GEOMETRY
void CDiskOperateDlg::OnButton1()
{
#ifdef  DISK_GET_GEOMETRY
hDevice = CreateFile(
"\\\\.\\I:",
// "\\\\.\\c:",
0,  
FILE_SHARE_READ | FILE_SHARE_WRITE,  
NULL,
OPEN_EXISTING,
0,  
NULL);  

if (hDevice == INVALID_HANDLE_VALUE)
{  
TRACE("Failed to open volume I! \n");
return RET;
}
else
{
TRACE("Successful to open I:! \n");
}

bResult = DeviceIoControl(
hDevice,
IOCTL_DISK_GET_DRIVE_GEOMETRY,  
NULL, 0,
&dgt,
sizeof(dgt),
&junk,
(LPOVERLAPPED)NULL);

if (!bResult)
{              
TRACE("IOCTL failed... \n");
CloseHandle(hDevice);    
return RET;
}

TRACE("IOCTL successful... \n");
OUTPUT_DISKGEOMETRY(dgt);

BOOL bRS;
ULARGE_INTEGER fba, tnob, tnofb;
bRS =GetDiskFreeSpaceEx("I:", &fba, &tnob, &tnofb);
if(bRS==FALSE)
{
TRACE("Failed to get size of the volume %s \n", "I:");
}

CloseHandle(hDevice);    
#endif  // #ifdef  DISK_GET_GEOMETRY
}

下面是用DeviceIoControl获取的磁盘数据
The number of cylinder of the volume is 3
Fixed hard disk media
The number of tracks per cylinder is 255
The number of sectors per track is 63
The number of bytes per sector is 512
计算磁盘总量为:
3*255*63*512 =23.5M

可我的优盘实际大小在磁盘管理中显示为: 31M
用 GetDiskFreeSpaceEx 获取的值是正确的

大虾解惑!

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