Fake USB stick or SD card? How to test the real capacity (and what to do)
It says 1 TB, Windows agrees, files copy fine, and then videos won't play and photos come back grey. That's a fake-capacity card, and a full write-and-read test is the only reliable check.
How the trick works
Every flash drive has a small controller that tells the computer how big it is. On a fake, that number has been edited: a 32 GB chip reports 1 TB. The file system happily formats 1 TB and the drive accepts writes past 32 GB, but the controller either drops them or wraps around and overwrites the start of the drive. Nothing warns you. Checking the size in File Explorer or Finder proves nothing, because it only repeats what the controller claims.
Signs you might have one
- The price is far below branded drives of the same size (a "2 TB" stick for the price of a meal).
- An unusual size on a tiny stick or card: 1-16 TB USB sticks and microSD cards larger than the biggest genuine ones on sale.
- Files copy without errors but later won't open, videos stop part-way, or photos show grey bands.
- Write speed is very low and gets slower as it fills.
- A camera or dashcam keeps reporting "card error" once the card is partly full.
Testing the real capacity
Why does only a full test work? A fake can only be caught once you write past its real size; small copies come back perfectly. Wirebook has a hands-on game that shows it with numbered marbles, good for explaining it to someone else.
Copy anything you need off the drive first. The tests write to every free byte, so a full test of a big drive takes hours on a slow stick.
| System | Tool | How |
|---|---|---|
| Windows | H2testw (free, small, long-standing) | Select target, choose "all available space", Write + Verify. A good drive ends with "Test finished without errors"; a fake shows how much data was lost and the real size. |
| Windows | ValiDrive (free) | Spot-checks positions across the whole drive in minutes, without filling it. Good for a quick yes/no; H2testw for the full picture. |
| Mac / Linux | f3 (Fight Flash Fraud): f3write then f3read | f3write /Volumes/STICK then f3read /Volumes/STICK (Linux: the mount path). Reports "Data OK" and "Data LOST" in GB. |
| Linux | f3probe (destructive) | sudo f3probe --destructive --time-ops /dev/sdX finds the real size in a minute or two but erases the drive. Double-check the device name. |
Reading the result
- No errors and the size matches: genuine capacity. The real size will still be a bit under the label, because labels use 1 GB = 1,000,000,000 bytes and computers often show GiB: a real 64 GB drive shows about 59.6 GB. The unit converter does that sum.
- Data lost after a certain point: fake. The point where errors start is roughly the real capacity.
- Errors scattered from the start: a failing or very poor-quality chip. Don't use it.
What to do with a fake
- Ask for a refund and report the listing. Marketplaces usually refund counterfeit storage, and test results are good evidence.
- Recover files you copied to it from the original source. Data written past the real capacity is gone; files that fit in the first part may still open.
- Don't trust it for anything important. Tools such as f3fix can shrink the partition to the real size so it stops losing data, but the chips in fakes are often low-grade and fail early. Treat it as disposable at best.
Buying genuine storage
Buy from the brand's own store or a large retailer's own stock rather than third-party marketplace sellers, be suspicious of prices well below the going rate, and test any new drive or card before trusting it with photos. For a microSD in a camera, also check the speed class matches what the camera needs.
Related
Write-protected USB stick or SD card · SD card not detected · SD card full but looks empty · USB transfer speed drops · SSD slower than advertised
Questions
How can I tell if a USB stick or SD card is fake?
Fill it with test data and read it back with H2testw or ValiDrive on Windows, or f3write and f3read on Mac and Linux. The size shown in Explorer or Finder is no proof, because a fake controller reports the false size.
Why do files disappear or get corrupted on my new SD card?
A fake-capacity card discards or overwrites anything written past its real size. Test it; scattered errors from the start mean a failing card instead.
Is a 2 TB USB stick for a few pounds real?
Almost certainly not. Genuine high-capacity flash costs far more; cheap multi-terabyte sticks are usually 32-64 GB chips with an edited controller.
Can I fix a fake capacity USB stick?
f3fix can shrink the partition to the real capacity so it stops losing data, but the chips are often low quality. Get a refund and don't rely on it.
Why does my genuine 64 GB card show 59.6 GB?
Labels count 1 GB as 1,000,000,000 bytes, while computers often count in GiB (1,073,741,824 bytes). That gap is normal and is not a sign of a fake.
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