Is this code from STM32 FATFS User manual (um1721 ) correct?

I am going through a codebase, that uses the FatFS library by Chan and probably developed using the CubeMX code generator. I can see the code given below there. It is also described in STM32 user manual UM1721

A structure is defined as follows:

typedef struct
{ 
  uint8_t                 is_initialized[_VOLUMES];
  Diskio_drvTypeDef       *drv[_VOLUMES];
  __IO uint8_t            nbr;

}Disk_drvTypeDef;

disk is A global variable defined as:

Disk_drvTypeDef  disk = {0};

A function is defined as follows:

uint8_t FATFS_LinkDriver(Diskio_drvTypeDef *drv, char *path)
{
  uint8_t ret = 1;
  uint8_t DiskNum = 0;
  if(disk.nbr <= _VOLUMES)
  {
    disk.is_initialized[disk.nbr] = 0;
    disk.drv[disk.nbr] = drv;  
    DiskNum = disk.nbr++;
    path[0] = DiskNum + '0';
    path[1] = ':';
    path[2] = '/';
    path[3] = 0;
    ret = 0;
  }
  return ret;
}

My question is what happens when the disk.nbr == _VOLUMES, and I call the FATFS_LinkDriver function. Won't it try to place drv into disk.drv[_VOLUMES]? But the disk.drv is of size _VOLUMES. So shouldn't the maximum index be 1 less than that? Am I missing something?

This code invokes undefined behavior in this line:

disk.drv[disk.nbr] = drv;  

when disk.nbr == _VOLUMES.

Is this code from STM32 FATFS User manual (um1721 ) correct?

No, it is not. General remark - there is no guarantee that the code found in the documentation will be correct.