The asker is bringing up the SDIO peripheral on an STM32F407 to write raw sectors to a microSD card in 4-bit mode. They're stuck on the standard SD initialization handshake — specifically ACMD41 (SD_SEND_OP_COND), which is the command that negotiates voltage and polls the card until it exits the busy state.

Why this is trickier than it looks: The SD physical-layer spec requires a very specific dance before ACMD41 will succeed, and any misstep silently produces a card that ignores you or returns a CRC error:

• CMD0 (GO_IDLE_STATE) at ≤400 kHz clock — the card must be in identification mode.
• CMD8 (SEND_IF_COND) with pattern 0x1AA — mandatory for SDHC/SDXC cards. If you skip it, the card assumes you're a legacy host and will refuse HCS=1.
• ACMD41 is really two commands: CMD55 (APP_CMD) immediately followed by CMD41. You must send CMD55 before every ACMD41 retry, not just once.
• The argument to CMD41 must set the HCS bit (bit 30) if CMD8 succeeded, and include the correct VDD window (typically 0x00FF8000).

Direction toward a solution: Without the code, the most likely culprits on STM32F4 SDIO are:

• Response type mismatch. ACMD41 returns R3, which has no CRC. If SDIO_CMD is configured for a short response with CRC check enabled, the hardware sets CCRCFAIL — you must ignore that flag for R3 and read SDIO_RESP1 anyway.
• Polling loop. Bit 31 of the OCR is the "power-up busy" flag. It starts at 0 and flips to 1 when the card is ready. Loop CMD55+ACMD41 until bit 31 is set, with a ~1 second timeout.
• Clock and pull-ups. Init must happen at ≤400 kHz. Bump to 25 MHz only after CMD3. Also verify external pull-ups on DAT0..DAT3 and CMD — the STM32F4 Discovery boards don't provide these on breakout headers.
• CMD55 RCA argument. Before the card has an RCA (during init), CMD55's argument must be 0x00000000. After identification, it must be RCA << 16.

Gotchas: A common trap is enabling the SDIO clock output (CLKEN) only when sending a command — the card needs 74+ clocks of idle activity before CMD0. Also, some microSD cards are unusually picky about VDD window bits; if you send 0x40000000 alone (just HCS), older cards return an invalid response.