File content as of revision 3:35ac9ee7d2d6:

// Original
//      https://os.mbed.com/users/jeonbc/code/stm32-disco-example/
//
// Modified by K.Arai
//      July 25th, 2019
//

#include "select_program.h"
//#define EXAMPLE_7_USB_MSD
//#ifdef EXAMPLE_7_USB_MSD
#if 0

#include <stdlib.h>

#include "mbed.h"
#include "LCD_DISCO_F769NI.h"
#include "USBHostMSD.h"
#include "FATFileSystem.h"

LCD_DISCO_F769NI lcd;

DigitalOut led_green(LED1);
DigitalOut led_orange(LED2);
DigitalOut led_red(LED3);
DigitalOut led_blue(LED4);

Serial pc(SERIAL_TX, SERIAL_RX, 115200);

USBHostMSD msd;
FATFileSystem fs("usb");

void msd_task(void const *)
{
    int i = 0;
    int err;

    printf("wait for usb memory stick insertion\r\n");
    while(1) {
        //pc.printf("line:%d\r\n", __LINE__);
        wait_ms(500);
        // try to connect a MSD device
        while(!msd.connect()) {
            pc.printf("line:%d\r\n", __LINE__);
            wait_ms(500);
        }
        if (fs.mount(&msd) != 0) {
            pc.printf("line:%d\r\n", __LINE__);
            continue;
        } else {
            pc.printf("file system mounted\n");
        }

        pc.printf("line:%d\r\n", __LINE__);
        if  (!msd.connect()) {
            pc.printf("line:%d\r\n", __LINE__);
            continue;
        }

        // in a loop, append a file
        // if the device is disconnected, we try to connect it again

        // append a file
        File file;
        err = file.open(&fs, "test1.txt", O_WRONLY | O_CREAT |O_APPEND);

        pc.printf("line:%d\r\n", __LINE__);
        if (err == 0) {
            char tmp[100];
            sprintf(tmp,"Hello fun USB stick  World: %d!\r\n", i++);
            file.write(tmp,strlen(tmp));
            sprintf(tmp,"Goodbye World!\r\n");
            file.write(tmp,strlen(tmp));
            file.close();
        } else {
            pc.printf("FILE == NULL\r\n");
        }
        ThisThread::sleep_for(500);
        pc.printf("again\n");
        // if device disconnected, try to connect again
        while (msd.connected()) {
            wait_ms(500);
        }
        while (fs.unmount() < 0) {
            wait_ms(500);
            pc.printf("unmount\n");
        }
    }
}

int main()
{
    pc.baud(115200);

    pc.printf("\x1b[2J\x1b[H %s\r\n %s %s (UTC)\r\n",
              __FILE__, __DATE__, __TIME__);
    lcd.DisplayStringAt(0, LINE(1), (uint8_t *)"MBED EXAMPLE", CENTER_MODE);
    ThisThread::sleep_for(500);

    Thread msdTask(msd_task, NULL, osPriorityNormal, 1024 * 6);
    pc.printf("line:%d\r\n", __LINE__);

    lcd.Clear(LCD_COLOR_BLUE);
    lcd.SetBackColor(LCD_COLOR_BLUE);
    lcd.SetTextColor(LCD_COLOR_WHITE);

    BSP_LCD_SetFont(&Font24);
    lcd.DisplayStringAt(0, LINE(7), (uint8_t *)"HAVE FUN !!!", CENTER_MODE);

    while(true) {
        // WARNING: LEDs are OFF
        led_green = 1;
        led_orange = 1;
        led_red = 1;
        led_blue = 1;
        ThisThread::sleep_for(200);
        // WARNING: LEDs are ON
        led_green = 0;
        led_orange = 0;
        led_red = 0;
        led_blue = 0;
        ThisThread::sleep_for(1000);
    }
}

#endif