Just off phone with Sound Devices tech support again, confirming information mentioned earlier on this thread. The CF and SD cards are not back up to each other, but they both write the same data at the same time. On his machine he hit record, then pulled the SD card, received grey boxed "SD warning I/O error", machine kept recording onto the CF. Full file good on CF, nothing written to the SD. So if my Transcend (no longer approved) SD card had failed, then I should have the full recording on the CF. I did not. Both CF and SD had same six minutes of 18 minute recording. So he can only speculate 1) the buffer was overloaded, but then I should of received a "overflow I/O warning", or 2) a hardware issue above the buffer. He's never seen this before. So no satisfactory answer. Normally I would suspect my own operator error, but in this particular case I recall looking down at the screen, seeing the machine in record and the time rolling, then reporting the 18 minutes to the producer. Thanks for everyone's help.