Fake SD cards and corrupted videos: how to spot and test them

By the FixMP4 engineering team · October 7, 2026 · 6 min read

Some of the most confusing "corrupted video" cases are not corruption at all. The camera recorded perfectly; the memory card simply threw the data away. Counterfeit cards are common on marketplaces, and they are designed to look fine until you fill them.

How a fake-capacity card works

A fake card contains a small, cheap memory chip, say 32 GB, with its controller reprogrammed to report a much larger capacity such as 256 GB or 1 TB. The computer and camera believe the advertised size. Writes within the real capacity work. Writes beyond it are accepted but silently discarded or wrapped around to overwrite earlier data.

The result: videos recorded after the card's real capacity is reached are full of zeros or of other files' data, and earlier files can be overwritten too.

Symptoms

• Recent clips will not play, while older ones on the same card are fine.
• Files have plausible sizes but contain long stretches of zeros.
• Photos show grey bottom halves or repeating blocks.
• The camera reports errors only when the card is nearly "full".
• The card's speed is far below its label, causing recordings to stop at high bit rates.

How to test a card

Do this with every new card before trusting it, and with any card you suspect:

1. Copy everything off the card first.
2. Windows: run H2testw, which fills the card with test data and verifies it.
3. Mac and Linux: run F3 (f3write, then f3read), which does the same.
4. A genuine card verifies its full capacity without errors. A fake card reports a real capacity far below the label.
5. Check speed with the same tools or with a benchmark; a V30 card should sustain around 30 MB/s writes.

What can be saved

Data written within the card's real capacity can be recovered and, if the recording was cut off, repaired. Data beyond it was never stored. The order of work is:

1. Stop using the card.
2. Copy all files, then image the card if the footage matters.
3. Scan with a recovery tool such as PhotoRec; see recover video from a corrupted SD card.
4. For MP4 or MOV files that contain real frames but will not open, rebuild the index with the repair tool using a reference recorded on the same device.

A repaired clip from a fake card may stop early or contain frozen sections where the data was lost. The free preview shows exactly what survived before you pay.

Zero-filled files are a strong sign

If a file has a normal size but its content is entirely zeros, the card never stored the data. You can check with any hex viewer. Our page on 0-byte MP4 files covers this and the related empty-file case.

Buying cards safely

• Buy from the manufacturer's store or well-known retailers.
• Be suspicious of high capacities at low prices, especially from unknown marketplace sellers.
• Check packaging, serial numbers and the manufacturer's verification service if one exists.
• Test every new card before use, as above.

Fake versus failing cards

Genuine cards also wear out, especially in dashcams and security cameras that write constantly. Worn cards show errors more randomly, often across the whole card, and may become read-only. Test results and the card's history usually tell the two apart. Either way, retire the card.

Summary

A fake card loses data by design. Test new cards, buy from reputable sellers, and if footage is already lost, recover and repair what was stored within the real capacity. Device-specific advice is in our guides, for example dashcams and drones.

• SD cards
• counterfeit
• H2testw
• F3
• data loss