Blackbox creates LOGxxxxx.BFL but file remains 0 KB on MicoAir H743 V2

Describe the bug

Blackbox creates the LOGS directory and a LOGxxxxx.BFL file on the SD card, but the BFL file remains 0 KB and contains no recorded data.

The SD card itself is detected correctly and the filesystem reports "FS: Ready".

This occurs on a MicoAir H743 V2 with Betaflight 2026.6.1.

To Reproduce

1. FC: MicoAir H743 V2 (STM32H743), target MICOAIR743V2
2. Flash Betaflight 2026.6.1
3. Use a 4 GB microSD card formatted as FAT32 with MBR.
4. Insert the SD card into the FC.
5. Check "sd_info" and confirm that the filesystem reports "FS: Ready".
6. Set Blackbox device to SDCARD.
7. Set Blackbox mode to ALWAYS.
8. Set Blackbox sample rate to 1/4.
9. Disconnect USB and power the FC from the flight battery only.
10. Run the quad for about 30 seconds with the propellers removed.
11. Power off the FC and remove the SD card.
12. Open the SD card on a PC.

Result: The LOGS directory is created and a LOGxxxxx.BFL file is created, but the BFL file remains 0 KB.

Expected behavior

The LOGxxxxx.BFL file should contain the recorded Blackbox data and should not remain at 0 KB.

After recording and properly closing the log, the file should have a non-zero file size and should be readable by the Betaflight Blackbox Explorer.

Support ID

115e45c8-9168-412e-97ba-82c2ac318b66

Flight controller

MicoAir H743 V2

Other components

4 GB microSD card, FAT32, MBR MicoAir MTF-02 Optical Flow & Range Sensor Futaba T16IZ transmitter RadioMaster R81 V2 S-FHSS receiver

How are the different components wired up (including port information)

Radio receiver: RadioMaster R81 V2 S-FHSS, connected to the flight controller via SBUS.

MicoAir MTF-02 Optical Flow & Range Sensor: connected to the flight controller via UART.

The onboard microSD card is used for Blackbox logging.

Add any other context about the problem that you think might be relevant here

Betaflight version: 2026.6.1 Target: MICOAIR743V2 Build: 6dbc4218f MCU: STM32H743, 480 MHz

SD card:

• 4 GB microSD
• FAT32
• MBR
• SD card is detected correctly
• sd_info reports "FS: Ready"
• status reports the SD card as approximately 3932160 kB

Blackbox:

• Device: SDCARD
• Mode: ALWAYS
• Sample rate: 1/4
• Debug mode: BLACKBOX_OUTPUT

Observed behavior: The SD card is recognized and the filesystem is ready. The LOGS directory is created successfully. A LOGxxxxx.BFL file is also created successfully. However, the BFL file remains 0 KB and contains no recorded data.

I also tested with the USB cable disconnected and the FC powered only by the flight battery. The BFL file still remained 0 KB.

The SD card can be read by a PC card reader.