I recently used Gemini to help with some dashcam videos that weren't being saved properly. I was sure most of the data were there but the files wouldn't play in VLC or MPC, so I asked Gemini.

It suggested various things to try, and after pasting in the error messages each time it suggested more and more radical things. Eventually it suggested a program called Untrunc, where you give it a working video file as a reference and then the file that's faulty and as if by magic it worked!

Just wanted to mention this in case anyone else is struggling to get FFmpeg to repair a file.

Interesting - I took a look at how this works and why it needs a reference, and the answer is the usual one of the 'moov atom': a critical piece of metadata. Lots of programs output it at the end of the file, but that makes it vulnerable to truncation, and for streaming it's useful to move it to the beginning of the file. ffmpeg refers to this as "fast start".