Cannot mount SD using FatFS on STM32 - sd-card

Question

I'm trying to write to a MicroSD-Card using STM32F405 chip.
The pins are connected correctly and each pin on the MicroSD-Card slot can be written to by using HAL_GPIO_WritePin. (Messured with ossciloscope) I'm using CubeMX to generate the init code for TrueStudio, so hopefully everything is ok there as well. But when I run the following code, f_mount returns FR_DISK_ERR. The MicroSD-Card can be written to and read from. If I use a different Device number, i.e. "1:", I get FR_INVALID_DRIVE
So my question is: what could cause FR_DISK_ERR except a faulty MicroSD-Card?
Here is my code so far:
int main(void)
{
/* USER CODE BEGIN 1 */
/* USER CODE END 1 */
/* MCU Configuration----------------------------------------------------------*/
/* 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_SDIO_SD_Init();
MX_FATFS_Init();
/* USER CODE BEGIN WHILE */
FATFS fileSystem;
FIL testFile;
uint8_t testBuffer[16] = "SD write success";
UINT testBytes;
FRESULT res;
while((res = f_mount(&fileSystem, SD_MOUNT_PATH, 1)) != FR_OK){
printf("%d", res); //used to debug res, only for TrueStudio Debugger
}
uint8_t path[13] = "testfile.txt";
path[12] = '\0';
res = f_open(&testFile, (char*)path, FA_WRITE | FA_CREATE_ALWAYS);
HAL_GPIO_WritePin(GPIOA, GPIO_PIN_6, GPIO_PIN_RESET);
res = f_write(&testFile, testBuffer, 16, &testBytes);
HAL_GPIO_WritePin(GPIOA, GPIO_PIN_7, GPIO_PIN_RESET);
res = f_close(&testFile);
HAL_GPIO_WritePin(GPIOB, GPIO_PIN_0, GPIO_PIN_RESET);
}
in MX_FATFS_Init() FATFS_LinkDriver(&SD_Driver, SD_Path) is called and returns 0.

Answer 1

Without the SD_MOUNT_PATH macro, does the f_mount call look like this:
f_mount(&fileSystem, "0:", 1) ?'
Are you saying that f_open, f_write return FR_OK even if f_mount fails?!
FR_DISK_ERR usually means disk_read() or disk_write() failed. Try giving 100ms or 1 sec delay before using f_mount() and between between fatfs function calls.