This fake USB test checks whether a flash drive or memory card really holds the capacity printed on it. It is the method used by H2testw and F3: the tester fills the drive's free space with files of known data, reads every byte back, and compares. A genuine drive returns all of it. A fake drive returns damaged or overwritten data once the test passes its real size, and the drive map shows exactly where. It works with USB flash drives, SD and microSD cards and external drives, in Chrome, Edge and Opera on Windows, Mac, Linux and ChromeOS.
How to test a flash drive for fake capacity
- Copy anything important off the drive. An empty drive gives the clearest result.
- Plug it into a port on the computer itself and press Choose. Pick the drive in the folder dialog and allow editing when asked.
- Select the size printed on the label, for example 128 GB or 2 TB.
- Choose Full test to check everything, or Quick check to test only the first 4, 16 or 64 GB.
- Press Start test and leave the computer awake. While it writes, the tester rereads samples of older data, so a fake is usually caught well before the drive is full.
- When writing ends, eject the drive, unplug it and plug it back in. The check starts by itself and reads everything straight from the drive.
- Read the verdict. Save report downloads the figures as a text file, useful for a refund claim.
What the results mean
| Readout | What it shows | Genuine drive |
|---|---|---|
| Written | Free space filled with test data | Close to the label size, minus formatting |
| Checked OK | Test data that read back exactly as written | Same as Written |
| Lost or wrong | Data missing, changed or overwritten | 0.0 GB |
| Write speed | Average write speed while filling | Matches the drive's class |
| Read speed | Average read speed while checking | Several times the write speed |
A genuine "128 GB" drive offers about 122 to 128 GB of free space when empty, because drive makers count 1 GB as 1,000,000,000 bytes, keep some memory for the drive itself, and the file system takes a little. Windows shows the same space as about 114 to 119 "GB", since it counts in units of 1,073,741,824 bytes.
| Verdict | Meaning |
|---|---|
| Genuine | All free space kept its data and it matches the label |
| Fake or faulty | A large part of the data was lost or overwritten. The tester shows the real working size |
| Errors | A small amount of data was damaged. The drive or its connection is unreliable |
| Quick check passed | The tested part works, the rest was not tested |
How fake flash drives work
A fake drive contains a small memory chip and a controller programmed to report a much bigger size. A stick sold as 1 TB might hold 32 GB. The computer believes the reported size, so it lets you copy far more than fits. Writes past the real size either wrap around and overwrite data at the start of the drive, or are thrown away. Nothing warns you while copying. You find out later when photos open half grey, videos stop partway, files show zero bytes or folder names turn into gibberish.
The tester recognizes both kinds. Every 512 byte block of test data carries its own position, so when a block comes back holding data meant for another position, the tester knows the drive wrapped around and can tell how far apart the two positions are, which is close to the drive's real size.
Signs of a fake USB drive or memory card
- The price. A 2 TB stick for the price of a meal is fake. Real high capacity drives cost close to what an SSD of the same size costs.
- An unknown brand or a cloned one. Fake SanDisk, Samsung and Kingston packaging is common on marketplaces. Buy from the brand's own store or a large retailer's own stock.
- Very slow writes. Many fakes write at 2 to 10 MB/s. The usb speed tester measures it in a minute.
- Damaged files after big copies. Small files open fine, then large copies come back broken. That is the drive running out of real memory.
- The drive drops off. Some fakes disconnect or lose their folders once filled past their real size, and Windows may then report errors. The usb device not recognized guide covers genuine port and driver problems.
The test says the drive is fake
- Stop using it for anything you care about.
- Ask for a refund. Marketplaces accept fake capacity claims, and the saved report plus a screenshot of the drive map is good evidence.
- If you keep it, limit it to its real size so it can no longer overwrite data. On Linux, f3fix from the F3 tools creates a partition of the real size. On Windows, open Disk Management, delete the drive's volume, then create a new simple volume no larger than the working size from the test.
- Run the full test again after resizing. Everything should now check OK, though fake drives often use low grade memory that fails early.
Errors on a drive that is not fake
A small number of bad blocks scattered across the map, rather than a solid block of orange at the end, points to a failing or worn drive, or a bad connection. Try the test on another port without a hub or extension cable. If errors appear again in different places, the memory is wearing out. SD cards used in dash cams and security cameras often end up this way. For a memory card, also check its speed on the sd card speed test page, since worn cards slow down before they fail.
Quick check or full test?
A quick check is enough to catch most fakes, which hold 8 to 64 GB whatever the label says. Only a full test proves the whole drive. Approximate times for a full test, writing and reading back the whole drive:
| Drive size | Writes at 10 MB/s | Writes at 30 MB/s | Writes at 100 MB/s |
|---|---|---|---|
| 32 GB | About 1 h 10 min | About 25 min | About 7 min |
| 128 GB | About 4 h 45 min | About 1 h 35 min | About 28 min |
| 1 TB | About 37 h | About 12 h | About 3 h 45 min |
A fake drive usually stops the test early, because the samples of older data fail soon after the real memory runs out.
Desktop tools that do the same job
H2testw on Windows and f3write and f3read on Linux and Mac use the same fill and verify method. They are free and trusted. F3 also has f3probe, and Gibson Research's ValiDrive runs a fast spot check on Windows, both by reading and writing the raw drive, which a browser is not allowed to do. That is why this tester fills the drive through ordinary files, and why it asks you to replug the drive: your computer keeps a copy of recently written data in memory, and reading that copy could hide a fake. Replugging clears it.
The tester cannot see space taken by files already on the drive, so a drive with files on it reports less free space than its label. It cannot test a drive the computer does not mount, and it needs Chrome, Edge or Opera on a computer. For an internal drive's speed rather than its capacity, use the ssd speed test.
Fake USB test questions
How long does a fake USB test take?
A full test writes the whole free space and reads it back, so it depends on size and speed. A 32 GB stick that writes at 30 MB/s takes about 25 minutes. A 128 GB stick at 10 MB/s takes nearly 5 hours. Fakes are usually caught much sooner, because the test checks older data while it writes and stops once it finds damage.
Will this fake flash drive test erase my files?
No. It only writes its own files into free space and never opens or deletes yours. On a fake drive, though, writing past its real size can overwrite files that are already on it, so copy anything important off first.
Can a fake USB drive be fixed?
Its real memory cannot be made bigger, but you can stop it from losing data by making a partition that matches the real size. Use f3fix on Linux, or delete the volume in Windows Disk Management and create a new one no larger than the working capacity the test reported. Keep it for unimportant files only.
Why does a fake drive show the full size in File Explorer?
The drive's controller has been programmed to report the size printed on the label. Windows, macOS and phones believe it, so the size, the free space and even copying files look normal until the data is read back.
Is a 2 TB flash drive for a few dollars real?
Almost never. Genuine 2 TB flash drives and 1 TB microSD cards exist from major brands, but they cost far more. Cheap sticks and cards sold as 1 TB, 2 TB or 16 TB usually hold 8 to 64 GB.
Does a quick check prove a drive is genuine?
No. It proves the part it tested works. Most fakes hold 8 to 64 GB, so a quick check of the first 16 or 64 GB catches many of them, but only a full test checks every gigabyte.