unmount and mount again fail

The first time it's ok to mount the sd card and read/write files.
```
 MX_SPI3_Init();
 MX_FATFS_Init();
HAL_Delay(1000);
init_sdcard();
```

then I remove sd card and insert sd card, use the same code(above) to init sd card. It returns 
f_mount error (1)

Is there any extra work to do?
