Recovering your files
You have an "SD error" on the GoPro, you have accidentally deleted some important files or your SD card is simply unreadable ?
There is a powerful solution for you : .
And they also provide a complete documentation here: http://www.cgsecurity.org/wiki/TestDisk_Step_By_Step
Assertion failed at core/pushencoder.cpp(429) - Invalid state
The "Assertion failed at core/pushencoder.cpp(429) - Invalid state" error is sent by the encoder meaning that your settings don't fit in a "standard MP4 preset" or that your output folder doesn't exists. This also could be due to a high FPS, a bad size (too large or not multiple of 16), a too large bitrate... You also have to make sure that the framerate you ask for fit into the output standard (PAL or NTSC)
The first thing to check is that your output folder exists and that you have the write rights on this folder. You also have to make sure that your settings fit in a standard level of the MP4/h.264 norm, see the section "Levels" of the wiki article: http://en.wikipedia.org/wiki/H.264/MPEG-4_AVC