How to check that a microSD card isn't counterfeit

A counterfeit microSD card doesn't break down: it lies. Its controller is reprogrammed to report 512 GB when the chip actually holds 64. The computer sees it as a 512 GB card, formatting confirms 512 GB, and the first files copy over without a single error. The trap closes later, once writes go beyond the real capacity: they overwrite data that is already there, without the slightest message. On a retro console, it's your saves that disappear, and nobody connects it to the card. Only one test settles the question.

What a fake-capacity card really is

A microSD card reports its size to the system through its internal registers, and the system takes it at its word: it has no way of counting the chips. A counterfeit exploits exactly that. The controller reports an inflated capacity, often four to eight times the real one, and keeps reporting it even after a full format.

What happens next depends on the trick used. Most often, addresses beyond the real memory loop back to the start of the card: writing at the 100 GB address amounts to writing at the 4 GB address and destroying whatever was there. In other cases, data written beyond capacity goes nowhere and reads back as zeros. No error is reported during writing.

A second, lesser-known family: the capacity is real, but the card is built with downgraded chips rejected at the factory. It loses sectors over the weeks. The test described below detects both cases.

Why it's worse on a retro console

A console's card has two zones. A large static zone, written once — system, game files, box art — and a small live zone, rewritten every session: saves, save states, the menu database, configuration. The static zone is what fills the card, so it is the one that spills past the real capacity. And on a card that loops back, some of the writes from the live zone land right on data that is already there.

So the symptom is never “the card is dead”. It's a game that restarts from scratch, a save that won't load, box art turned into a square of pixels, an emulator that always crashes at the same spot. You suspect the firmware, reinstall, reformat, and the card goes around again. Our article on the causes of a game that won't launch lists the card among the suspects, and our guide to saves shows where they are stored on each system.

Warning signs at the time of purchase

None of these signs proves anything on its own. Two or three together justify a test before you use the card.

• The price. Flash memory has a price per gigabyte. A 512 GB card listed at a third of the price of an equivalent branded card has no manufacturing secret. Be careful, though: some counterfeits are sold at full price so as not to raise suspicion. A normal price guarantees nothing.
• The packaging and the card. Poor-quality printing, fonts that don't match the manufacturer's, a label that is off-center or peeling, spelling mistakes, no barcode, a blister pack without a seal. On branded cards, the absence of a serial number printed on the back is a warning sign.
• The advertised capacity. The consumer market currently tops out at 2 TB, and those cards are expensive and rare. Anything above that is fake.
• The brand. Unknown brands, and above all well-known brands that have never made memory: a phone maker's logo on a microSD card is a warning sign in itself. A part number you can't find on the manufacturer's website doesn't exist.
• The seller. On marketplaces, look at who actually ships the item: a product sold by a third party on an official-looking listing doesn't commit the retailer.

Cards supplied “free” with a console remain the most common source of unpleasant surprises: they aren't what the sale is about, and nobody has checked them. The criteria that really matter are in our guide to choosing your microSD card.

Why the capacity shown by your computer proves nothing

Everyone's reflex: plug in the card, read “476 GB free” in the file explorer, and conclude it's fine. That information comes from the card itself. A counterfeit reports exactly the figure you hope to see, and will still report it after formatting. Disk information tools that merely read these registers are no better.

Incidentally, the gap between the printed capacity and the one shown by Windows has nothing to do with fraud. Manufacturers count in decimal gigabytes, one billion bytes, while the system displays gibibytes, 1,073,741,824 bytes. A 128 GB card therefore shows up at around 119 GB. Copying a single large file proves nothing either, as long as the total written stays below the real capacity.

The only reliable method: write the whole card, then read it back

The principle fits in one sentence: you fill the card to the very last byte with known data, then read everything back and compare. An honest card returns exactly what it was given; a rigged card returns zeros, data from another block or noise beyond its real capacity. It takes time, and no shortcut gives the same certainty.

On Windows: H2testw

H2testw is a free utility from the German magazine c't, written by Harald Bögeholz. It doesn't need installing: you unzip the archive and run the executable. The interface is available in German or English.

1. Format the card, or empty it: the test writes to the free space.
2. Launch the program, choose English, then Select target and pick the card's drive letter.
3. Leave the all available space option ticked. A partial test proves nothing.
4. Click Write + Verify. The program writes 1 GB .h2w files until the card is full, then reads everything back and compares.
5. When it's done, delete the .h2w files and reformat the card.

Don't unplug anything during the process and turn off sleep mode: it invalidates the result.

On Linux and macOS: f3write and f3read

F3, for “Fight Flash Fraud”, does the same thing from the command line: sudo apt install f3 on Debian and Ubuntu, brew install f3 on macOS with Homebrew. The card must be formatted and mounted; you pass the program the mount point, not the raw device.

• Write: f3write /media/user/CARD_NAME on Linux, f3write /Volumes/CARD_NAME on macOS.
• Read back: f3read followed by the same path.

f3write also creates 1 GB .h2w files and displays the average write speed; f3read reads them back and displays a summary. Neither needs special privileges. The project also provides f3probe, much faster but Linux-only: it works on the unmounted device, requires administrator rights and destroys the contents.

The quick check that doesn't replace the test

On Windows, ValiDrive gives a first answer in a few minutes. It doesn't fill the card: it probes 576 regions of 4 kilobytes spread across the entire reported capacity, about 2.4 MB in total, and displays a color grid. Its author presents it as a quick spot check, not a thorough test. It's for sorting, not for drawing conclusions.

How long to allow

The duration depends on the capacity, the card and the reader, never on the software: capacity divided by write speed, plus capacity divided by read speed. The rough figures below assume a USB 3 reader with a V30 card (30 MB/s write, 80 MB/s read), then a USB 2 reader or an entry-level card (12 MB/s and 30 MB/s). Adjust for your own hardware.

Start the test in the evening and let the machine work. The reader matters more than any setting: those built into laptops often top out well below what the card can do.

Reading the result

H2testw displays Test finished without errors when all is well, along with the volume checked and the measured speeds. With F3, f3read shows a Data OK line equal to everything that was written, Data LOST at zero, and the Corrupted, Slightly changed and Overwritten counters at zero. A healthy card doesn't give an in-between result.

If it fails, H2testw reports The media is likely to be defective and details how many megabytes are correct and how many are lost, distinguishing overwritten, slightly changed and corrupted sectors, and “aliased” memory, meaning addresses that loop back onto others. F3 says the same thing with Data LOST, Overwritten and Corrupted.

Reading it is straightforward. If the correct volume matches a smaller standard capacity — around 8, 16, 32 or 64 GB on a card sold as 512 GB — you have a textbook fake-capacity card. If it is close to the total with a few megabytes lost, the capacity is probably real and it's the chips that are bad. The practical verdict is the same: this card won't be getting your saves.

The card is fake: what to do

Don't reformat it hoping to salvage the situation. No format can fix a capacity reported by the controller, and the next time it fills up it will destroy the same data in the same place.

1. Keep the evidence. A screenshot of the report, the packaging, the invoice. An H2testw or F3 report is concrete proof.
2. Request a refund, stating in writing that the product doesn't match its description. For a distance purchase from a professional seller, the 14-day right of withdrawal applies automatically; beyond that, the legal guarantee of conformity takes over.
3. Go through the marketplace or the payment provider if the seller doesn't respond. Platform dispute procedures handle this case routinely.
4. Report it. For a seller based in France, SignalConso, the service run by the French DGCCRF. Elsewhere in the European Union, the European Consumer Centre helps with cross-border disputes free of charge.

Then set it aside rather than recycling it as a spare card: a fake-capacity card used “just to carry files” always ends up swallowing one. If, on the other hand, the test comes back clean, prepare it as usual and put your files back according to your firmware's folder structure.

What we do with the cards we sell

We apply the method described here. Every microSD card sold by us goes through a full write and read-back check before shipping: filled to the end, read back in full, and sent only if the report is clean. It's the only way to be certain of what we ship.

That doesn't exempt you from checking a card bought elsewhere, and it doesn't protect against any future failure: a genuine card is still a consumable that wears out. The rule that truly protects your progress doesn't change: back up your saves to your computer regularly. The cards checked before shipping are grouped in our microSD cards section.

Frequently asked questions