I have had several occasions (arducopter 3.3.3) where no dataflash log file was produced during a flight (or several flights).
I have verified that the sd card is usable, get the expected beeps when copter is powered up.
I tried a very short arm, move servos a bit with RC Transmitter, disarm - and I got an SD Card (dataflash) log file. Re-installed the card and flew - with no log files produced!
It is sometimes critical to have these log files (photo tagging if automatic tagging fails!) or post flight analysis.
log bitmask set to 456703 - normally logs a lot of data.
Thanks for any suggestions / help.