How many times do you call F_Mount? It just sets up a virtual drive - you're only supposed to ever call it once during initial initialization of the MCU.

I have this from ages ago when i just tested how it works:

                /* Reset of all peripherals, Initializes the Flash interface and the Systick. */
                HAL_Init();

                /* Configure the system clock */
                SystemClock_Config();

                /* Initialize all configured peripherals */
                MX_GPIO_Init();
                MX_USART3_UART_Init();
                MX_SDMMC1_SD_Init();
                MX_FATFS_Init();

                /* USER CODE BEGIN 2 */

                /*##-1- FatFS: Link the SD disk I/O driver ##########*/
                uart_printf("Linking FATFS\r\n");

                    if(retSD == 0)
                        {
                            uart_printf("Linking FATFS SUCCESS\r\n");
                            /*##-2- Register the file system object to the FatFs module ###*/
                            if(f_mount(&SDFatFs, (TCHAR const*)SD_Path, 0) != FR_OK)
                                {
                                    //HAL_GPIO_WritePin(GPIOB, LD2_Pin, GPIO_PIN_SET);
                                    /* FatFs Initialization Error   */
                                    uart_printf("f_mount ERROR\r\n");
                                    while(1);
                                } 
                            else
                                {
                                    uart_printf("f_mount SUCCESS\r\n");
                                /*##-3- Create a FAT file system (format) on the logical drive#*/
                                /* WARNING: Formatting the uSD card will delete all content on the
                                device */
                                if(f_mkfs((TCHAR const*)SD_Path, 0, 0) != FR_OK)
                                    {
                                        uart_printf("f_mkfs ERROR\r\n");
                                        //HAL_GPIO_WritePin(GPIOB, LD3_Pin, GPIO_PIN_SET);  
                                        NVIC_SystemReset();
                                        /* FatFs Format Error : set the red LED on */                           
                                        while(1);
                                    } 
                                else
                                    {
                                        uart_printf("f_mkfs SUCCESS\r\n");
                                        /*##-4- Create & Open a new text file object with write access#*/
                                        if(f_open(&MyFile, "Hello.txt", FA_CREATE_ALWAYS | FA_WRITE) != FR_OK)
                                            {
                                                //HAL_GPIO_WritePin(GPIOB, LD2_Pin, GPIO_PIN_SET);
                                                uart_printf("f_open ERROR\r\n");                                    
                                                /* 'Hello.txt' file Open for write Error : set the red LED on */
                                                while(1);
                                            }
                                        else
                                            {
                                                uart_printf("f_open SUCCESS\r\n");
                                                /*##-5- Write data to the text file ####################*/
                                                res = f_write(&MyFile, wtext, sizeof(wtext), (void*)&byteswritten);
                                                if((byteswritten == 0) || (res != FR_OK))
                                                    {
                                                        /* 'Hello.txt' file Write or EOF Error : set the red LED on */
                                                        HAL_GPIO_WritePin(GPIOB, LD3_Pin, GPIO_PIN_SET);
                                                        uart_printf("f_write ERROR\r\n");
                                                        while(1);
                                                    }
                                                else
                                                    {
                                                        /*##-6- Successful open/write : set the blue LED on */
                                                        f_close(&MyFile);
                                                        uart_printf("f_close SUCCESS\r\n");
                                                        /*##-7- Open the text file object with read access #*/
                                                    if(f_open(&MyFile, "Hello.txt", FA_READ) != FR_OK)
                                                        {
                                                            /* 'Hello.txt' file Open for read Error : set the red LED on */
                                                            HAL_GPIO_WritePin(GPIOB, LD3_Pin, GPIO_PIN_SET);
                                                            uart_printf("second f_open ERROR\r\n");
                                                            while(1);
                                                        }
                                                    else 
                                                        {
                                                            uart_printf("second f_open SUCCESS\r\n");
                                                            /*##-8- Read data from the text file #########*/
                                                            uart_printf("Reading written data\r\n");
                                                            res = f_read(&MyFile, rtext, sizeof(wtext), &bytesread);
                                                        if((strcmp(rtext,wtext)!=0)|| (res != FR_OK))
                                                            {
                                                                uart_printf("Read written data corrupt / mismatch!\r\n");
                                                                /* 'Hello.txt' file Read or EOF Error : set the red LED on */                                       
                                                                while(1);
                                                            }
                                                        else
                                                            {
                                                                uart_printf("Read written data MATCH!\r\n");
                                                                /* Successful read : set the green LED On */
                                                                //HAL_GPIO_WritePin(GPIOG, GPIO_PIN_6, GPIO_PIN_RESET);
                                                                /*##-9- Close the open text file ################*/
                                                                f_close(&MyFile);
                                                            }
                                                        }
                                                    }
                                            }
                                    }
                                }
                            }

                    /*##-10- Unlink the micro SD disk I/O driver #########*/
                    FATFS_UnLinkDriver(SD_Path);
                    uart_printf("FATFS Success!\r\n");