Not able to send a .bin file from computer to the micro-controller via UART, STM32F415 - terminal

Question

I am trying to send a file(.hex file) from my computer to the micro-controller's internal flash. For time being I am using Hercules terminal to send file. My UART responds to the data sent.
My internal flash memory sector is 128Kbytes and my file is about 50Kbytes , so space is not a problem.
While sending .hex file upto a certain point in file the data gets transferred but after a while it stops . I don't understand why.
To slow it down , I have tried my UART baud rate form 115200 to 2400.
Below is the code:
while(1)
{
i = 0;
int c;
char str[256];
printf("\n> ");
do
{
c = fgetc(stdin);
if(c=='\n')
break;
if(c!=-1)
{
str[i++] = c;
delay(10);
}
}while(1);
//str[i]='\0';
//printf("Got..%s\n",str);
int j = 0;
while(j < i-1)
{
uint64_t data;
uint64_t *pData = (uint64_t*)(str + j); //
//data = *((uint64_t*)&str[i]);
//++pData;
data = *pData;
if (HAL_FLASH_Program(TYPEPROGRAM_BYTE, start_address, data) != HAL_OK) {
HAL_FLASH_Lock();
}else
{
//printf("\nSuccess: Writing a byte at (%x) ==> %c ",start_address,*((char*)&data));
}
delay(10);
//data++;
start_address=start_address+1;
j++;
}
}
Below I am attaching my Hercules terminal image :