What to do if F405 V3 flight controller's blackbox can't be recognized or generate log files of 0KB or a few tens of KB?

Step 1 Check if your microSD card is at least C10 level

Check if your microSD card has a read/write speed of 10MB/s by using this popular speed test tool--h2testw.

Betaflight requires SDSC (standard) or SDHC (high capacity) type, and does not support SDXC (extended capacity) type which many high-speed U3 cards belong to. You can use a Class 10 SD card that is less than or equal to 32GB, but the flight controller can only recognize up to 4GB. And it must be formatted using FAT16 or FAT32 (recommended) format. It is recommended to use a third-party formatting tool（https://www.sdcard.org/downloads/formatter/） recommended by Betaflight to format your SD card, and select "Overwrite format" before formatting.

Ther SD card quality can vary, and it is recommended to purchase the 8GB SD card that has been tested by us with both read and write speeds of 15MB/s or higher: https://www.speedybee.com/8gb-class10-high-speed-microsd-card-for-blackbox-logging-tested-by-speedybee/

Step 2: Make sure to wait for about 10 seconds after disarming the quad before removing the battery.

Otherwise, the flight controller may not be able to save the blackbox file correctly.

Step 3: Try reformatting the SD card.

Note that the format must be FAT/FAT32. It is recommended to use FAT32. Please refer to: How to format the Blackbox storage chip on SpeedyBee F405 V3? In this step, it is important to note that after formatting, exit the USB drive, unplug the flight controller, and then connect it to the computer again. Do not open the Betaflight configurator first (if the Betaflight configurator is already open, close it first), wait for about 10 seconds, and then open Betaflight configurator to view the blackbox page. This is because the flight controller needs some time to generate a placeholder file FREESPAC.E in the SD card.

Step 4: Try using a lower blackbox write rate.

You can try setting the blackbox rate to the default 1/4 or lower, such as 1/8, 1/16, etc. to see if there is any improvement.