A card printed as 128 GB or 256 GB, sold cheaply, that accepts files happily and then loses them, corrupts photos, or fails to boot a device is the classic fake-card pattern. Fake cards are real flash chips reprogrammed to report far more capacity than they physically contain. Files written beyond the real capacity wrap around and overwrite earlier files, or simply vanish. No imaging tool can fix that, and no formatting trick can create the missing flash. The only reliable defence is a full write-and-verify test before you trust the card with anything important.
Win32 Disk Imager makes fake cards especially painful, because a Write overwrites the entire destination and a Read copies the whole device sector by sector. A fake card may appear to accept a full-device write, then fail to boot or lose data when the device tries to read back sectors that were never really stored. If you have had a write succeed and then a boot fail, or a backup that restores with corruption, test the card before blaming the image — see write succeeded but won’t boot and slow write speed, both of which share fake cards as a common cause.
How to test with h2testw on Windows
- Back up anything on the card first, if it is still readable. The test overwrites the whole card, so anything on it will be destroyed. Copy what you can before testing.
- Download h2testw from a reputable source and run it as administrator. Select the card’s drive letter carefully — writing test data to the wrong drive destroys that drive’s contents, just as a Win32 Disk Imager write would.
- Choose Write + Verify and test the full capacity, not a small sample. A partial test can miss a fake card whose real capacity is larger than the sample but smaller than printed. Only a full test is conclusive.
- Wait for the result. A genuine card completes with no errors and reports the full tested capacity. A fake card reports data loss, verification errors, or a real capacity far below printed — for example, a card sold as 128 GB that only holds 16 GB.
- Do not trust a card that fails, even partially. If the test shows any significant shortfall, the card is not usable for reliable storage, even for files smaller than its real capacity, because its controller is already lying about its geometry.
F3 on Linux and macOS
F3, short for Fight Flash Fraud, provides the same idea on Linux and macOS: f3write fills the card, f3read verifies it, and the report shows real versus claimed capacity. If you work across systems, F3 is the cross-platform counterpart to h2testw. The principle is identical — write everything, read everything back, believe only the verified result — and a quick format or a capacity reading in a file manager proves nothing on its own, because the fake controller reports the fake size to the operating system too.
What to do with the result
- Pass: the card is genuine as far as a full test can prove. It can still wear out or fail later, so keep backups, but capacity fraud is ruled out.
- Fail: stop using the card for anything important, return it if you can, and replace it from a reputable seller. Do not keep it as a spare for small files; its controller cannot be trusted.
- Uncertain or interrupted: if the test was interrupted by a reader disconnect, retry with a different reader and a direct port before concluding. A reader fault can mimic a card fault — see Error 121.
When you replace a failed card, buy on sustained reliability, not the cheapest price per gigabyte. A durable endurance microSD card (Sponsored) and a reliable USB card reader (Sponsored) remove the two hardware variables that most often confuse fake-card diagnosis: a slow or flaky reader can make even a genuine card look bad. A fresh copy of Win32 Disk Imager is on the download page, and the troubleshooting hub covers the symptoms that lead people to test in the first place.
When to stop and switch tools
Stop testing a card that has failed a full h2testw or F3 test on two readers. No second test, no format, and no different imaging tool will make its missing capacity appear, because the capacity was never there. Switching tools is useful only when a test was interrupted by hardware — a disconnect or a reader fault — and you need a clean rerun. Once a full, uninterrupted test has failed, the verdict is final: replace the card.
FAQ
What is h2testw and what does it do?
h2testw writes test data to the whole card and reads it back, verifying that the card really holds as much as it claims. It is a free Windows tool widely used to expose fake-capacity cards.
How long does a full test take?
It depends on card size and speed. A large card on a slow reader can take hours, because the test must write and read every sector to be conclusive. A quick partial test cannot prove a card is genuine.
Can a fake card be fixed by formatting?
No. Formatting cannot create capacity that is not physically there. A card that fails a full test should be replaced, not reformatted and trusted.
Is F3 an alternative if I also use Linux or Mac?
Yes. F3 provides similar write-and-verify testing on Linux and macOS, while h2testw is the usual Windows choice. Win32 Disk Imager itself is Windows-only, so h2testw fits the same PC workflow.