Why isn't copying a few files enough to prove a card is real?

Because a fake card writes normally until you exceed its true capacity. It keeps the folder and file names visible but overwrites the actual data in a rolling way, so early files look present but won't open. You only discover the loss later, often after the return window closes. A full write-and-verify is the only check that exercises the whole claimed capacity.

What does an H2testw result actually tell me?

H2testw writes 1 GB blocks of known data across the entire card, then reads them back. "Test finished without errors" means every block came back intact — the capacity and basic integrity are real. If it reports some gigabytes OK and the rest as corrupted, the OK figure is the card's true size and the rest was a lie. It also shows real read/write speed, which is often far below the box claim.

Which tool should I use on Mac or Linux, or on a phone?

H2testw is Windows-only. On Mac and Linux use F3 (Fight Flash Fraud) — f3write then f3read do the same full write-and-verify. On Android, SD Insight reads the card's manufacturer and true reported data, and apps like ValiDrive (Windows) spot-check capacity faster. The principle is identical everywhere: write across the full claimed size and read it back.

How to test a microSD or USB drive for fake capacity (H2testw and F3)

Fake flash is one of the few scams that passes a casual test on purpose. The card mounts, shows the advertised size, and accepts the first files you copy. The fraud only surfaces later, when the data you trusted it with turns out to be gone. Here is exactly how the trick works, the one test that catches it every time, and the buy-side signals worth reading before you spend anything.

What a fake card actually is

A counterfeit card is a small, cheap flash chip — often 8, 16 or 32 GB — with its controller firmware rewired to report a much larger size to the host [1]. The operating system believes the lie and lets you write up to the fake number.

The damaging part is what happens when you pass the real capacity. The card doesn't error out. It keeps the directory and file names intact, but the underlying data wraps around and overwrites what you wrote earlier [1]. So the first photos still appear in the file list — they just won't open. By the time you notice, the footage, the backup or the shoot is already gone, and usually the refund window with it.

This is why a quick check is worthless: copying one file, or even filling a quarter of the card, exercises only the genuine portion. The fraud lives in the gap between the claimed size and the real one.

The one test that works: write the whole card, read it back

Every reliable check does the same thing — fill the entire claimed capacity with known data, then read it all back and compare. If every byte returns intact, the capacity is real. If the back half comes back as garbage, you've found the chip's true size.

On Windows — H2testw [1]:

Format the card (FAT32/exFAT) and make sure it's empty.
Open H2testw, select the drive, choose all available space, click Write + Verify.
Let it run to the end. A full card takes a while — budget on the order of an hour or more per 128 GB depending on real speed. Don't interrupt it.
Read the result.

On macOS / Linux — F3 [2]: run f3write to fill the card, then f3read to verify. Same logic, same verdict.

On Android — SD Insight reads the card's true manufacturer and reported capacity; ValiDrive (Windows) [4] does a faster sampled spot-check when you don't have time for a full pass.

How to read the result

The verdict is blunt:

Result	Meaning
"Test finished without errors" — OK to the full size	Capacity and integrity are genuine.
e.g. "14.8 GB OK, 102 GB corrupted" on a 128 GB card	It's really a ~16 GB chip lying about its size. The OK number is the truth.
Errors scattered throughout	Failing or counterfeit flash — don't trust it with anything.

One honest subtlety so you don't false-alarm: real usable capacity is always slightly below the label. A genuine "128 GB" card reports about 119 GiB because the industry counts 1 GB as 1,000,000,000 bytes while your OS counts in 1,073,741,824-byte GiB — that's normal, and we cover the math in why a 128 GB card shows 119 GB. A fake is nothing like that small gap; it collapses to a fraction of the label.

H2testw also prints the real read and write speed it measured. Compare that to the box. A card sold as "high speed" that verifies at single-digit MB/s write is either mislabeled or built on the cheapest flash — worth knowing even when the capacity is genuine. (For what the speed markings actually promise, see speed classes decoded.)

Read these red flags before you even buy

Testing catches a fake after it arrives. These signals catch it before money leaves your account:

Price too good to be true. Flash has a floor set by the NAND inside it. Any 1 TB card for under ~US$20, or a "256 GB" for a couple of dollars, is fake — there is no legitimate way to hit that price.
Loose card in a baggie from an unknown seller, no retail packaging, no part number you can look up on the maker's site.
Commingled marketplace inventory. On large marketplaces, stock from many sellers is pooled, so a genuine listing can still ship you a counterfeit that someone else returned — buying from the brand's own store is not a guarantee.
"Lifetime warranty" with no specifics. A real endurance card states a TBW or rated hours and a concrete warranty term. A vague "lifetime" with no numbers is marketing, not a spec — and for continuous recording, endurance is a separate thing you choose on purpose (high-endurance vs consumer).

Where this leaves a B2B buyer

If you resell cards, the counterfeit problem is your problem twice: once on your own cost, and again every time an end customer brings back a card that failed. The defence is a supply chain with no place for fakes to enter — a known factory source rather than pooled marketplace stock, and product you can verify yourself.

That's the standard we hold ourselves to: every batch is 100% H2testw-verified for full capacity at the factory, and we ship with the test report so you can confirm it — or run your own pass and check. We'd rather you test the card than take our word for it. Tell us the capacities and volumes you move and we'll send samples to verify before any order.