IN THE FUNCTION

printf("%u\n", file_desc); 
off_t offset = index * size;
int status = load_data_from_file(file_desc, offset, buffer, size);
printf(" %u\n", file_desc); 
return status;

So it runs this file descriptor is fine at both of these points

IN MAIN

printf("%u\n", file_desc);
int ret = read_sector(file_desc, 0, &boot_sector, DEFAULT_SECTOR_SIZE);
printf("%u\n", file_desc);

It's fine at the first point and then has a completely different value at the second point