Posted on February 18, 2017 at 23:05Not something I can debug for you. You'd want to verify how the MicroSD card is connected on your board, and the pull-ups on the pins. Assuming this is an original STM32F4-DISCO board and you've wired a card/socket yourself. You'd want to build some test code to verify that the SDIO peripheral can successfully read and write the media.
You would also want to Eject and safely remove the MSC from the PC to ensure the data is flushed to the media from the caches.
