STM32F746G-DISCO, FLASH

Sorry leader, but this is too little information. I'm using 20+ of those boards for years and they never did that to me.

FLASH_SECTOR_4 != FLASH_SECTOR_3 ?

Keep in mind that flash wears out easily. Max. number of guaranteed cycles (10k) is in the data sheets. There is a EEPROM emulation library, see AN4894 "EEPROM emulation techniques and software for STM32 microcontrollers" and X-CUBE-EEPROM library doing some wear-leveling and other house keeping with a more user oriented API.

Something along the lines of this should work

/* Uncomment the following define to select the FLASH Dual Bank mode,
 or keep it commented to select the FLASH Single Bank mode */
 
#define DUAL_BANK
 
/* Base address of the Flash sectors */
#if defined(DUAL_BANK)
#define ADDR_FLASH_SECTOR_0 ((uint32_t)0x08000000) /* Base address of Sector 0, 16 Kbytes */
#define ADDR_FLASH_SECTOR_1 ((uint32_t)0x08004000) /* Base address of Sector 1, 16 Kbytes */
#define ADDR_FLASH_SECTOR_2 ((uint32_t)0x08008000) /* Base address of Sector 2, 16 Kbytes */
#define ADDR_FLASH_SECTOR_3 ((uint32_t)0x0800C000) /* Base address of Sector 3, 16 Kbytes */
#define ADDR_FLASH_SECTOR_4 ((uint32_t)0x08010000) /* Base address of Sector 4, 64 Kbytes */
#define ADDR_FLASH_SECTOR_5 ((uint32_t)0x08020000) /* Base address of Sector 5, 128 Kbytes */
#define ADDR_FLASH_SECTOR_6 ((uint32_t)0x08040000) /* Base address of Sector 6, 128 Kbytes */
#define ADDR_FLASH_SECTOR_7 ((uint32_t)0x08060000) /* Base address of Sector 7, 128 Kbytes */
#define ADDR_FLASH_SECTOR_8 ((uint32_t)0x08080000) /* Base address of Sector 8, 128 Kbytes */
#define ADDR_FLASH_SECTOR_9 ((uint32_t)0x080A0000) /* Base address of Sector 9, 128 Kbytes */
#define ADDR_FLASH_SECTOR_10 ((uint32_t)0x080C0000) /* Base address of Sector 10, 128 Kbytes */
#define ADDR_FLASH_SECTOR_11 ((uint32_t)0x080E0000) /* Base address of Sector 11, 128 Kbytes */
#define ADDR_FLASH_SECTOR_12 ((uint32_t)0x08100000) /* Base address of Sector 12, 16 Kbytes */
#define ADDR_FLASH_SECTOR_13 ((uint32_t)0x08104000) /* Base address of Sector 13, 16 Kbytes */
#define ADDR_FLASH_SECTOR_14 ((uint32_t)0x08108000) /* Base address of Sector 14, 16 Kbytes */
#define ADDR_FLASH_SECTOR_15 ((uint32_t)0x0810C000) /* Base address of Sector 15, 16 Kbytes */
#define ADDR_FLASH_SECTOR_16 ((uint32_t)0x08110000) /* Base address of Sector 16, 64 Kbytes */
#define ADDR_FLASH_SECTOR_17 ((uint32_t)0x08120000) /* Base address of Sector 17, 128 Kbytes */
#define ADDR_FLASH_SECTOR_18 ((uint32_t)0x08140000) /* Base address of Sector 18, 128 Kbytes */
#define ADDR_FLASH_SECTOR_19 ((uint32_t)0x08160000) /* Base address of Sector 19, 128 Kbytes */
#define ADDR_FLASH_SECTOR_20 ((uint32_t)0x08180000) /* Base address of Sector 20, 128 Kbytes */
#define ADDR_FLASH_SECTOR_21 ((uint32_t)0x081A0000) /* Base address of Sector 21, 128 Kbytes */
#define ADDR_FLASH_SECTOR_22 ((uint32_t)0x081C0000) /* Base address of Sector 22, 128 Kbytes */
#define ADDR_FLASH_SECTOR_23 ((uint32_t)0x081E0000) /* Base address of Sector 23, 128 Kbytes */
#else
#define ADDR_FLASH_SECTOR_0 ((uint32_t)0x08000000) /* Base address of Sector 0, 32 Kbytes */
#define ADDR_FLASH_SECTOR_1 ((uint32_t)0x08008000) /* Base address of Sector 1, 32 Kbytes */
#define ADDR_FLASH_SECTOR_2 ((uint32_t)0x08010000) /* Base address of Sector 2, 32 Kbytes */
#define ADDR_FLASH_SECTOR_3 ((uint32_t)0x08018000) /* Base address of Sector 3, 32 Kbytes */
#define ADDR_FLASH_SECTOR_4 ((uint32_t)0x08020000) /* Base address of Sector 4, 128 Kbytes */
#define ADDR_FLASH_SECTOR_5 ((uint32_t)0x08040000) /* Base address of Sector 5, 256 Kbytes */
#define ADDR_FLASH_SECTOR_6 ((uint32_t)0x08080000) /* Base address of Sector 6, 256 Kbytes */
#define ADDR_FLASH_SECTOR_7 ((uint32_t)0x080C0000) /* Base address of Sector 7, 256 Kbytes */
#define ADDR_FLASH_SECTOR_8 ((uint32_t)0x08100000) /* Base address of Sector 8, 256 Kbytes */
#define ADDR_FLASH_SECTOR_9 ((uint32_t)0x08140000) /* Base address of Sector 9, 256 Kbytes */
#define ADDR_FLASH_SECTOR_10 ((uint32_t)0x08180000) /* Base address of Sector 10, 256 Kbytes */
#define ADDR_FLASH_SECTOR_11 ((uint32_t)0x081C0000) /* Base address of Sector 11, 256 Kbytes */
#endif /* DUAL_BANK */
 
uint32_t ReadValue(void)
{
 return(*(uint32_t *)ADDR_FLASH_SECTOR_3);
}
 
void WriteValue(uint32_t Value)
{
 FLASH_EraseInitTypeDef EraseInitStruct = {0};
 uint32_t SECTORError = 0;
 
 if (ReadValue() == Value) return; // Already at current value
 
 /* Unlock the Flash to enable the flash control register access *************/
 HAL_FLASH_Unlock();
 
 /* Fill EraseInit structure*/
 EraseInitStruct.TypeErase = FLASH_TYPEERASE_SECTORS;
 EraseInitStruct.VoltageRange = FLASH_VOLTAGE_RANGE_3;
 EraseInitStruct.Sector = FLASH_SECTOR_3;
 EraseInitStruct.NbSectors = 1;
 
 /* Note: If an erase operation in Flash memory also concerns data in the data or instruction cache,
 you have to make sure that these data are rewritten before they are accessed during code
 execution. If this cannot be done safely, it is recommended to flush the caches by setting the
 DCRST and ICRST bits in the FLASH_CR register. */
 if (HAL_FLASHEx_Erase(&EraseInitStruct, &SECTORError) != HAL_OK)
 {
 ErrorHandler();
 }
 
 if (HAL_FLASH_Program(FLASH_TYPEPROGRAM_WORD, ADDR_FLASH_SECTOR_3, Value) != HAL_OK)
 {
 ErrorHandler();
 }
 
 /* Lock the Flash to disable the flash control register access (recommended
 to protect the FLASH memory against possible unwanted operation) *********/
 HAL_FLASH_Lock();
} // sourcer32@gmail.com