"moov atom not found": what it means and what can fix it
The error means an MP4 or MOV is missing its index, usually because recording stopped suddenly. When it can be repaired, when it cannot, and what to try.
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In short: "moov atom not found" means the MP4 or MOV has no index describing where its frames are. It happens when a recording is cut off before it finishes. If the index is only damaged, Repair a Broken Video can rebuild it. If it was never written, the file needs a specialist tool and a working video from the same camera.
What the moov atom is
An MP4 is written as a stream of picture and sound data plus an index, called the moov atom, that says where every frame is. Cameras and phones usually write the index last, when you press stop. If the battery dies, the card is pulled or the app crashes, the data is there and the index is not, and every player refuses the file.
What this site can fix
Repair a Broken Video copies the streams it can read into a fresh container, which rebuilds a damaged or inconsistent index. It fixes files that will not seek, show the wrong length or stop partway.
It cannot recreate an index that was never written. Without the moov atom, nothing can read the file at all, including the repair tool, so it will report that it cannot open the file.
When the index is missing entirely
The established approach is a recovery tool such as untrunc, which takes a healthy recording from the same camera with the same settings and uses its index as a template. It runs on your own computer and often recovers most of the footage.
Stop recording to the card in the meantime. New files can overwrite the data you are trying to save.
Questions
Can a video without a moov atom be recovered?
Often, yes, with a reference-based tool such as untrunc and a working file from the same camera. An ordinary converter cannot, because it has no index to read.