Data Recovery Case File · Cameras, Drones & Cards · The Index That Was Never Written

Corrupted SD Card Video: the Index That Was Never Written

His enquiry was precise enough to contain its own solution, and it describes a fault that behaves quite unlike the card failures elsewhere in this archive. "I have a microSD card with a corrupted video file from a dashcam. The dashcam lost power from the engine and cut off, which caused the corruption. Other files are accessible, and Windows recognises the file but just can't open it." Every detail there is diagnostic. The other files open, so the card is fine — this is not a card recovery at all. Windows can see the file and read its size, so the data is present. And the cause is exactly as he describes: the recording was interrupted. What follows is the reason that combination produces an unplayable file, and it comes down to a single structural fact about video that explains a very large number of "corrupted footage" cases: a video file's index is written at the end.

The decode: why the index comes last, and what that means for his clip

How a video file is actually written: a camera recording to a card does not write a finished file — it writes a stream of compressed frames as they are captured, appending continuously for as long as recording lasts. Only when recording stops does the camera perform finalisation: it writes the container's index — the table that says where each frame begins, how many there are, what the duration is, and how audio aligns with video — and completes the header. That index is what a player reads first in order to know how to play the file. So the structure is: hours of content, then a small but essential table at the end describing it.

Why a power cut produces exactly his symptom: if the camera dies mid-recording, the frames written up to that instant are safely on the card — but finalisation never happens. The result is a file with a full body and no index, and a header that still claims a recording is in progress. Windows sees a file of substantial size, because the frames are genuinely there. Every player refuses it, because the first thing they look for does not exist. And because the fault is entirely inside that one file, nothing else on the card is affected — which is precisely what he observed. This is the single most common cause of "corrupted" dashcam and action-camera footage, and it is why the word corrupted is slightly misleading: nothing was damaged. Something was never written.

What the repair involves: the fix is reconstruction rather than rescue. The card is imaged write-blocked so the original is never modified, and the affected file's frame stream is parsed directly — each frame located and measured — so a correct index can be built from what is actually present. A healthy clip from the same camera provides the structural reference: same encoder, same resolution, same frame rate, same container conventions, which makes the rebuilt index match what the camera would itself have written. The header is completed, the file closed properly, and then the only test that counts: it is played from beginning to end.

The honest limit: stated plainly, because it usually applies. The very end of the recording — typically the last seconds before power was lost — may be a partial frame or an incomplete write, so a rebuilt clip can be a moment or two shorter than the event, and the final fraction of a second may show artefacts. Everything up to that point plays normally. For a dashcam clip captured around an incident, that distinction can matter, and it is described accurately rather than glossed over.

On the bench

The card was imaged write-blocked behind the DeepSpar USB Stabilizer 10Gb — routine, since the card was healthy, but the original was never going to be altered. On the copy, the affected clip's frame stream was parsed from the beginning: every frame located, its offset and length recorded, and the container's index rebuilt from the real contents rather than guessed. An intact clip from the same camera served as the structural template, so the reconstructed index and completed header matched the device's own conventions. The repaired file was then validated the only way that means anything — played through from start to finish, with the tail examined honestly and its exact end point reported — and delivered alongside the untouched original.

The outcome

The interrupted clip rebuilt from its own frame data and validated by full playback, with the truncated tail described precisely. Free assessment, one fixed written figure including VAT, no recovery, no fee. The decode, for everyone holding footage that will not open: a video file's index is written when recording stops, so a camera that loses power mid-recording leaves the frames intact and the index missing — the file is unfinalised rather than damaged; that is why the clip shows a full size and no player will touch it, and why nothing else on the card is affected; and the repair rebuilds the index from the frames themselves, with an intact clip from the same camera as the reference. Expect the last second or two to be short — and expect the rest to play exactly as recorded.

Our case files are drawn from genuine enquiries received by our laboratory over the past ten years, anonymised to protect client confidentiality. Each one describes the diagnostic and recovery procedure our engineers apply to that fault, using the equipment listed.