Is That Download Actually Complete?

Why some broken files play and some do not

An MP4 keeps an index describing where everything is. Whether a truncated file opens depends entirely on where that index sat.

We tested both. A 30-second video cut to a third of its bytes with the index at the end produced moov atom not found and would not open at all. The same video prepared with the index at the front, cut identically, reported a duration of 30 seconds, opened happily, and threw partial file errors as it ran out of data.

The second row is the dangerous one. The file reports the right length because the index was written before the data was cut off — the metadata describes a video that is no longer entirely there.

The thirty-second check

1. Compare the file size to what was promised. A few per cent off is normal; a third of the expected size is a truncated download.
2. Skip to the last minute. Drag the playback position near the end rather than watching from the start. A partial file fails there and nowhere else.
3. Check the reported duration matches the source. If the platform said ten minutes and the file says four, it stopped early in a way that also broke the index.

Step two is the one that matters and the one nobody does. Playing the first ten seconds proves the file opens, which is a different question from whether it is whole.

Do it before the moment you need it. A file checked at home is a minor inconvenience if it is wrong; the same file discovered on a plane or in front of an audience is not recoverable.

The technical check, if you want certainty

ffprobe, which ships with ffmpeg, reads a file without playing it:

ffprobe -v error -show_entries format=duration,size -of default=nw=1 video.mp4

A whole file answers with a duration and a size. A badly truncated one reports moov atom not found instead — the index is missing, so there is nothing to read.

For the deceptive case, decode the whole thing without producing output:

ffmpeg -v error -i video.mp4 -f null -

Silence means the file decoded end to end. Any output — partial file, Invalid data, errors about frames — means it did not, whatever the duration claimed. On our test the streaming-optimised truncated file passed the first check and failed this one, which is exactly the pattern to catch.

What a partial file cannot be

Worth stating plainly because it saves time spent on the wrong idea.

It cannot be repaired. The missing data was never received. Tools advertising video repair rebuild an index so a file opens; they do not restore content, and the video still stops where the download did.

It cannot be resumed. A file assembled for you does not exist on a server to continue from, so the transfer restarts — covered in full here.

The part that arrived is genuinely fine. If you only needed the first two minutes, a truncated file containing them is usable. Nothing is wrong with the data that made it; there is simply less of it.

What causes truncation in the first place

• The connection dropped. Moving between networks, a weak signal, a VPN reconnecting — long transfers are exposed to all of it.
• Storage ran out. Phones abandon downloads quietly when space runs short — and the requirement is larger than the file.
• The device suspended the browser. Switching apps or locking the screen can stop a transfer on a phone.
• A server-side limit was reached. Size ceilings and processing timeouts cut a transfer mid-flight rather than after.

All four produce the same result and none announces itself. Which is why the check matters more than the cause: you cannot prevent every interruption, and you can always verify what you ended up with.

Building the habit into an archive

For anything you are keeping rather than watching immediately, verification belongs in the routine.

Check on arrival, not on use. The gap between downloading and needing a file is where the source disappears — deleted posts, closed accounts, expired VODs.

Record the expected size. A line in a text file beside the video makes the comparison possible months later, when nothing else remembers what it should have been.

Verify before deleting anything. The one irreversible step in archiving an account is the deletion, and it should come after the files are confirmed — the order of operations matters.

Re-download while you still can. A truncated file discovered today is a minor annoyance; the same file discovered after the source is gone is a permanent loss.