Fake Capacity: Testing the USB Drive or Memory Card From Your Mystery Box
A flash drive or memory card that says 128GB, shows 128GB on your computer, and holds 8GB is the most common fraud in cheap storage, and you will not notice until the files you copied on come back as noise. The fix is a free test that takes an hour and needs no skill. Here is how you run it on the drive from your box, before you trust it with anything.
How a Drive Can Lie About Its Size
Every flash drive and memory card has a small controller chip that tells the computer how large the storage is. That chip can be programmed to report any number. A factory takes a small chip, tells the controller to announce a large one, prints the large number on the case, and sells it at the large number's price. Your computer reads the controller and believes it, so Properties says 128GB. Only the first few gigabytes are real.
What makes this worse than a simple short measure is how it fails. You copy files onto the drive and the copy appears to succeed; the operating system writes past the real capacity and the controller silently discards or overwrites the data. The files show in the folder with the right names and sizes. When you open one, it is corrupt. By then the original may be gone. That is why you test before you use, not after. Whether to plug an unknown drive into your main machine at all is a separate question, covered in is it safe to plug in a USB gadget from a mystery box; do the test on the machine you chose there.
The Test That Cannot Be Fooled
- A drive's reported size comes from a programmable chip, so Properties in Windows or Get Info on a Mac proves nothing; only writing and reading back the whole drive does.
- Free tools exist for every system: H2testw on Windows, F3 on Mac and Linux, ValiDrive for a quick check. All of them fill the drive with known data and read it back.
- A drive that fails is misdescribed, which is a fault you can claim on; a drive that passes but is slow is the product working.
The only test that a fake cannot pass is to fill the entire drive with data whose content you know, then read every byte back and compare. If the drive is genuine, everything matches. If the drive is fake, the comparison fails at the point where the real chips ran out, and the tool tells you where that was. Three free tools do this.
H2testw, for Windows, is the standard. It is a small program from the German computer magazine c't, and it has been the reference test for fake flash for years. You point it at the drive, click Write + Verify, and wait. At the end it reports how much of the drive was good and how much was not.
F3, which stands for Fight Flash Fraud, is the open-source equivalent for Mac and Linux. Two commands, f3write and f3read, do the same fill-and-check.
ValiDrive, from Gibson Research, is a quick check for Windows that samples the drive rather than filling it, so it finishes in minutes. It catches most fakes and is a reasonable first pass, but a full fill is the only conclusive result.
How Long It Takes, and What the Result Means
| Drive label | Full test at 10MB/s | Full test at 30MB/s | Result | What it means |
|---|---|---|---|---|
| 32GB | About 1 hour | 20 minutes | All verified | Genuine; use it |
| 64GB | About 2 hours | 40 minutes | All verified | Genuine; use it |
| 128GB | About 4 hours | 80 minutes | Errors after 7.8GB | Fake; real size is about 8GB |
| 256GB | About 7 hours | 2.5 hours | Errors after 31GB | Fake; real size is about 32GB |
| Any | Verified, but write speed 3MB/s | Genuine but slow; not a fault | ||
| Any | Test cannot start; drive disconnects | Failing hardware; a fault |
Read the speeds as well as the sizes. A fake drive is usually slow as well as small, because the chips inside are the cheapest available. A genuine drive that is merely slow is a real product doing what its price allows, and you have no complaint. The size result is the one you act on.
Note: Test a memory card in a reader you already trust, not the one that came in the same box. A cheap reader can drop connections and produce errors that look like a fake card. If a card fails in the box reader, try it in your camera, your phone's slot, or a known-good reader before you conclude anything. A card that fails in two readers is the card.
Step by Step on Windows
- Download H2testw from the c't or heise site, not from a search result you do not recognise. It is a zip with one small program inside.
- Plug the drive in and note the drive letter. Move anything you want off it first; the test does not destroy data, but you should treat the drive as empty.
- Run H2testw, choose the drive, leave "all available space" selected, and click Write + Verify.
- Wait. Do not use the drive for anything else during the test. The write pass fills it; the verify pass reads it back.
- Read the report. "Test finished without errors" means genuine. Anything mentioning corrupted or overwritten sectors means fake, and the number of good bytes is the real size.
- Delete the test files it left on the drive, or format the drive, before you use it.
Screenshot the report either way. If the drive failed, that screenshot with the drive and its packaging in a photograph is the whole of your claim. The evidence habits are in how to document a mystery box on arrival.
What to Do With a Fake
A fake-capacity drive is a misdescribed product, which puts it on the fault side of the line drawn in can you return a mystery box. Report it to the seller inside the window with the test screenshot and the photographs, and say what you want: a replacement or a partial refund for that item. You are not asking for goodwill; you are reporting that 128GB was sold and 8GB delivered.
Then decide what to do with the drive. You can use it at its real size by creating a partition no larger than the verified good space; H2testw's number tells you where to stop. An 8GB drive is still an 8GB drive. Or you can dispose of it, which for a drive worth a few dollars is often the simpler answer. What you must not do is put anything you care about on it at the labelled size.
What This Means When You Order
You should treat any storage device in a mystery box as unverified until you have filled it, and you should ask the seller where their drives come from. A seller who sources from a named manufacturer and checks a sample can say so; a seller who buys unbranded "128GB" drives by the thousand cannot. Mewoo tests devices before dispatch and states what its boxes are built around; ask any seller the same. Then run the fill test the day the box lands, because the seller's window closes long before the day the drive would have failed on its own.
FAQ
Windows says the drive is 128GB. Is that not proof?
No. The figure in Properties is whatever the tiny controller inside was told to announce, and a counterfeiter sets that number in seconds. Nothing on your screen reflects how many memory chips are physically present. The only proof is writing known data across the whole drive and reading it back; the point where the read-back turns to garbage is the true capacity, and no fake survives that.
Which tool should I use to test a USB drive or SD card?
Pick by operating system. Windows users have had H2testw, published by c't magazine, as the reference for over a decade. Mac and Linux users get F3, two open-source commands that write and then read. Short on time? ValiDrive from Gibson Research probes a sample of the drive in minutes and exposes most fakes, though only a complete fill is final. None of the three costs anything.
How long does a full capacity test take?
Roughly the labelled size divided by the write speed, twice. A slow 64GB stick at 10MB/s needs about two hours to fill and read; a decent one at 30MB/s finishes in forty minutes. Fakes often stop early because the test hits the error point long before the labelled size. Start it before dinner, leave the drive alone, and read the report afterwards.
The test says the drive is fake. Can I still use it?
At its real size, yes. Create a single partition no larger than the number of good bytes the test reported and the drive behaves like the small honest drive it always was. Whether that is worth the bother for an 8GB stick is your call. What you should never do is keep using it at the labelled size, because everything written past the true limit is lost.
Can I get a refund for a fake-capacity drive from a mystery box?
Yes, on the basis that the goods were not as described: 128GB promised, 8GB supplied. Attach the tool's report as a screenshot, a picture of the stick beside its packet, and one sentence saying whether you want a replacement or money back for that item, all within the seller's window. Evidence of this kind is hard to argue with, and a seller who tries has revealed how little they check their supplier.
My memory card fails the test in the reader from the box. Is the card fake?
Suspect the reader before the card. Bargain readers lose contact partway through a long write, and the resulting errors are indistinguishable from a counterfeit card's. Run the test again with the card seated in your camera, a phone's slot, or a reader that has served you before. A pass there clears the card and condemns the reader. Two failures in two different readers is the card, and the second report is your proof.
Final Thoughts
The size a drive reports is a number someone typed into a chip, and the only test that beats it is filling the drive and reading it back. H2testw, F3 or ValiDrive will do that for free in about an hour. A drive that passes is yours to use; a drive that fails is misdescribed, and the screenshot is your claim. Run it the day the box arrives, before the window closes and before you trust the drive with anything.
If you want a box whose devices are checked before dispatch and whose categories are stated on the product page, start with the Triple Reveal Electronics Mystery Box (3 items) or the Party Electronics Mystery Box (6 items). Compare every tier side by side on the electronics mystery box category page.
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