The HAL requires strict aliasing optimisation to be turned on to function
correctly (at least for the SD card driver on F4 MCUs). This optimisation
was recently disabled with the addition of H7 support due to the H7 HAL
having errors with the strict aliasing optimisation enabled. But this is
now fixed in the latest stm32lib and so the optimisation can now be
re-enabled.
Thanks to @chuckbook for finding that there was a problem with the SD card
on F4 MCUs with the strict aliasing optimisation disabled.