I just put together a new build with Mamba DJI F405 MK2 FC/Stack. Whenever I connect FC to Betaflight it is always defaulting to DFU mode. I flashed firmware but still same, every time I have to go to 'Flash Firmware' option and manually click 'Exis DFU' option to convert to COMM3 port and connect normally. Due to this I cant even arm because the FC is always sitting in DFU bootloader mode when I connect Lipo. Is this due to issue in the boot switch or something can be done in setting?

This sounds like a stuck boot button. See if you are able to check continuity on that button with a multimeter.

If this is a brand new board I would put in a support ticket with the manufacturer. They might have encountered this before and know a fix for you, or be able to replace the board with a new one.

Yes it is brand new board as part of a stack, this morning I tested the button with a multimeter for continuity result came as negative, means button not sorted, only showing continuity after pressing button which is expected. Thinking to drop an email to seller. I tried flashing the firmware to latest, everything looks good but still always DFU. 

@rockford007
Hi did you find what the problem was? I got the same issue with mamba mk2 ?

Hi. I have same problem, FC starts in DFU mode. Button checked all ok. Perhaps somebody know solution? FC Speedybee F405v3