我想把STM32F4系列的设备对外表现为一个U盘，即在PC端接入USB，显示为U盘。我在cubemx配置了USB Device，以及MSC。目前暂时没有用任何外挂存储，单单设置这些是否能够对外表现为U盘呢?
加上存储的话，无非就是添加文件系统后对存储的操作，不知道理解是否正确?
现在问题是我在cubemx设置后，PC端显示设备描述符请求失败，我需要如何改动呢?

file:///C:\Users\windows8\AppData\Roaming\Tencent\Users\344055076\QQ\WinTemp\RichOle\I4E79~3_`B%WYHJCB`NYGKH.png

在usbd_storage_if.c 中，添加一些代码。

/**
  ******************************************************************************
  * @file           : usbd_storage_if.c
  * @brief          : Memory management layer
  ******************************************************************************
  * COPYRIGHT(c) 2016 STMicroelectronics
  *
  * Redistribution and use in source and binary forms, with or without modification,
  * are permitted provided that the following conditions are met:
  * 1. Redistributions of source code must retain the above copyright notice,
  * this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright notice,
  * this list of conditions and the following disclaimer in the documentation
  * and/or other materials provided with the distribution.
  * 3. Neither the name of STMicroelectronics nor the names of its contributors
  * may be used to endorse or promote products derived from this software
  * without specific prior written permission.
  *
  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
  * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
  * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
  * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
  * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  *
  ******************************************************************************
*/

/* Includes ------------------------------------------------------------------*/
#include "usbd_storage_if.h"
/* USER CODE BEGIN INCLUDE */
#include "fat16.h"
/* USER CODE END INCLUDE */

/** @addtogroup STM32_USB_OTG_DEVICE_LIBRARY
  * @{
  */

/** @defgroup USBD_STORAGE
  * @brief usbd core module
  * @{
  */

/** @defgroup USBD_STORAGE_Private_TypesDefinitions
  * @{
  */
/* USER CODE BEGIN PRIVATE_TYPES */
/* USER CODE END PRIVATE_TYPES */
/**
  * @}
  */

/** @defgroup USBD_STORAGE_Private_Defines
  * @{
  */
#define STORAGE_LUN_NBR                  1  
#define STORAGE_BLK_NBR                  0x100  
#define STORAGE_BLK_SIZ                  0x800

/* USER CODE BEGIN PRIVATE_DEFINES */
/* USER CODE END PRIVATE_DEFINES */
  
/**
  * @}
  */

/** @defgroup USBD_STORAGE_Private_Macros
  * @{
  */
/* USER CODE BEGIN PRIVATE_MACRO */
/* USER CODE END PRIVATE_MACRO */

/**
  * @}
  */

/** @defgroup USBD_STORAGE_IF_Private_Variables
  * @{
  */
/* USER CODE BEGIN INQUIRY_DATA_FS */
/* USB Mass storage Standard Inquiry Data */
const int8_t  STORAGE_Inquirydata_FS[] = {/* 36 */
  
  /* LUN 0 */
  0x00,               
  0x80,               
  0x02,               
  0x02,
  (STANDARD_INQUIRY_DATA_LEN - 5),
  0x00,
  0x00,       
  0x00,
  'S', 'T', 'M', ' ', ' ', ' ', ' ', ' ', /* Manufacturer : 8 bytes */
  'P', 'r', 'o', 'd', 'u', 'c', 't', ' ', /* Product      : 16 Bytes */
  ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ',
  '0', '.', '0' ,'1',                     /* Version      : 4 Bytes */
};
/* USER CODE END INQUIRY_DATA_FS */

/* USER CODE BEGIN PRIVATE_VARIABLES */
/* USER CODE END PRIVATE_VARIABLES */

/**
  * @}
  */

/** @defgroup USBD_STORAGE_IF_Exported_Variables
  * @{
  */
  extern USBD_HandleTypeDef hUsbDeviceFS;
/* USER CODE BEGIN EXPORTED_VARIABLES */
/* USER CODE END EXPORTED_VARIABLES */

/**
  * @}
  */
  
/** @defgroup USBD_STORAGE_Private_FunctionPrototypes
  * @{
  */
static int8_t STORAGE_Init_FS (uint8_t lun);
static int8_t STORAGE_GetCapacity_FS (uint8_t lun,
                           uint32_t *block_num,
                           uint16_t *block_size);
static int8_t  STORAGE_IsReady_FS (uint8_t lun);
static int8_t  STORAGE_IsWriteProtected_FS (uint8_t lun);
static int8_t STORAGE_Read_FS (uint8_t lun,
                        uint8_t *buf,
                        uint32_t blk_addr,
                        uint16_t blk_len);
static int8_t STORAGE_Write_FS (uint8_t lun,
                        uint8_t *buf,
                        uint32_t blk_addr,
                        uint16_t blk_len);
static int8_t STORAGE_GetMaxLun_FS (void);

/* USER CODE BEGIN PRIVATE_FUNCTIONS_DECLARATION */
/* USER CODE END PRIVATE_FUNCTIONS_DECLARATION */

/**
  * @}
  */
  
USBD_StorageTypeDef USBD_Storage_Interface_fops_FS =
{
  STORAGE_Init_FS,
  STORAGE_GetCapacity_FS,
  STORAGE_IsReady_FS,
  STORAGE_IsWriteProtected_FS,
  STORAGE_Read_FS,
  STORAGE_Write_FS,
  STORAGE_GetMaxLun_FS,
  (int8_t *)STORAGE_Inquirydata_FS,
};

/* Private functions ---------------------------------------------------------*/
/*******************************************************************************
* Function Name  : STORAGE_Init_FS
* Description    :
* Input          : None.
* Output         : None.
* Return         : None.
*******************************************************************************/
int8_t STORAGE_Init_FS (uint8_t lun)
{
  /* USER CODE BEGIN 2 */
  return (USBD_OK);
  /* USER CODE END 2 */
}

/*******************************************************************************
* Function Name  : STORAGE_GetCapacity_FS
* Description    :
* Input          : None.
* Output         : None.
* Return         : None.
*******************************************************************************/
int8_t STORAGE_GetCapacity_FS (uint8_t lun, uint32_t *block_num, uint16_t *block_size)
{
  /* USER CODE BEGIN 3 */   
  *block_num  = STORAGE_BLK_NBR;
  *block_size = STORAGE_BLK_SIZ;
  return (USBD_OK);
  /* USER CODE END 3 */
}

/*******************************************************************************
* Function Name  : STORAGE_IsReady_FS
* Description    :
* Input          : None.
* Output         : None.
* Return         : None.
*******************************************************************************/
int8_t  STORAGE_IsReady_FS (uint8_t lun)
{
  /* USER CODE BEGIN 4 */
  return (USBD_OK);
  /* USER CODE END 4 */
}

/*******************************************************************************
* Function Name  : STORAGE_IsWriteProtected_FS
* Description    :
* Input          : None.
* Output         : None.
* Return         : None.
*******************************************************************************/
int8_t  STORAGE_IsWriteProtected_FS (uint8_t lun)
{
  /* USER CODE BEGIN 5 */
  return (USBD_OK);
  /* USER CODE END 5 */
}

/*******************************************************************************
* Function Name  : STORAGE_Read_FS
* Description    :
* Input          : None.
* Output         : None.
* Return         : None.
*******************************************************************************/
int8_t STORAGE_Read_FS (uint8_t lun,
                        uint8_t *buf,
                        uint32_t blk_addr,                       
                        uint16_t blk_len)
{
  /* USER CODE BEGIN 6 */
  FATReadLBA(blk_addr * STORAGE_BLK_SIZ,(uint8_t*)buf, (uint32_t)blk_len * STORAGE_BLK_SIZ);
  return (USBD_OK);
  /* USER CODE END 6 */
}

/*******************************************************************************
* Function Name  : STORAGE_Write_FS
* Description    :
* Input          : None.
* Output         : None.
* Return         : None.
*******************************************************************************/
int8_t STORAGE_Write_FS (uint8_t lun,
                         uint8_t *buf,
                         uint32_t blk_addr,
                         uint16_t blk_len)
{
  /* USER CODE BEGIN 7 */
  FATWriteLBA(blk_addr * STORAGE_BLK_SIZ,(uint8_t*)buf, (uint32_t)blk_len * STORAGE_BLK_SIZ);
  return (USBD_OK);
  /* USER CODE END 7 */
}

/*******************************************************************************
* Function Name  : STORAGE_GetMaxLun_FS
* Description    :
* Input          : None.
* Output         : None.
* Return         : None.
*******************************************************************************/
int8_t STORAGE_GetMaxLun_FS (void)
{
  /* USER CODE BEGIN 8 */
  return (STORAGE_LUN_NBR - 1);
  /* USER CODE END 8 */
}

/* USER CODE BEGIN PRIVATE_FUNCTIONS_IMPLEMENTATION */
/* USER CODE END PRIVATE_FUNCTIONS_IMPLEMENTATION */

/**
  * @}
  */

/**
  * @}
  */  
/************************ (C) COPYRIGHT STMicroelectronics *****END OF FILE****/