How to Test a Fake SD Card: H2testw, F3 and FakeFlashTest

To test whether an SD or microSD card is fake, write files across its entire advertised capacity and read them back. On Windows, use H2testw or FakeFlashTest. On Mac and Linux, use F3. If you see errors or a smaller real capacity, the card is counterfeit.

Why is a fake SD card a problem?

A counterfeit card is programmed to tell your computer or phone that it holds, say, 256 GB when it holds far less. Everything looks normal until the real memory runs out. After that, the card overwrites old data or writes garbage, and you only find out when you open a photo or video that no longer exists.

That is why testing matters before the card goes into a camera, drone or phone. If you are still picking a model, our guide to microSD card speed ratings explains C10, U3, V30 and A2. Speed and real capacity are different things: a fake card can even feel fast.

What are the warning signs before you test?

According to MakeUseOf, some clues show up at purchase:

• A brand that doesn’t make cards: Xiaomi doesn’t make microSD cards, so a card with that name probably lacks the capacity on the label.
• Generic or unknown brands: many imitate SanDisk and Samsung and can have storage failures.
• Details that don’t match: the author found a “Verbatim 44082” card on Wish that matched a different official model, the 44010, which is a lower class.
• A price that’s too good: deals far below market often have strings attached.

None of these proves anything. Only a write and read test does.

What should you do before testing?

1. Back up everything on the card. A capacity test writes to all free space, and the F3 author recommends testing empty cards because, if the card is faulty, old files can be wiped.
2. Use a card reader or adapter, not a camera or other device. The F3 documentation advises against testing cards inside cameras.
3. Pick the right drive. Double-check the drive letter so you don’t run the test on the wrong disk.
4. Set aside time. A full test writes and reads the whole card, so it takes longer on big cards.

How do you test on Windows with H2testw?

H2testw is free and needs no installation. It fills all free space with files, then reads them back to compare.

1. Download H2testw, extract it and open the program.
2. Under “Select target,” choose the card’s drive (often E: or F:). Make sure it isn’t your computer’s drive.
3. Check “All available space” and leave “Endless verify” unchecked. For a quick first check, you can limit the size to something like 500 MB.
4. Click “Write + Verify” and wait.
5. Read the result. A genuine card ends with a message saying the test finished without errors. A fake one reports how much data was lost, corrupted or overwritten.

How do you test on Windows with FakeFlashTest?

FakeFlashTest has two options. “Quick Size Test” writes and reads 512 bytes in random segments of the card and is much faster. “Test Empty Space” checks all free space. Write or read failures show up in the program’s log. For a card you just bought, run the quick test first and, if it passes, do the full one.

How do you test on Mac and Linux with F3?

F3 (Fight Flash Fraud) is the option for anyone not on Windows. Per its documentation it runs from the command line, and there are graphical front ends: F3X on macOS and F3-qt on Linux. On Debian-based systems like Ubuntu, install it with sudo apt install f3. On a Mac with Homebrew, use brew install f3.

File test (f3write and f3read)

1. Plug in the card and note where it is mounted.
2. Run f3write /path/to/card/. It fills the card with 1 GB files.
3. Unplug and reconnect the card so f3read doesn’t read from the system cache.
4. Run f3read /path/to/card/.

f3read sorts sectors into four groups: ok, corrupted (lost or altered data), changed (right position but some flipped bits) and overwritten (valid sector, but read from the wrong position). Anything other than “ok” is bad news. The final speeds are estimates and can be capped by your reader or USB port.

Quick test on the device (f3probe)

f3probe is much faster on fake cards because it writes only a few blocks, but it needs admin rights:

1. Find the device with lsblk and pick the whole disk (for example sdb), not a partition (sdb1). The documentation warns that choosing the wrong device can damage your computer.
2. Run sudo f3probe --time-ops /dev/sdb. The --destructive option speeds things up but doesn’t restore the original contents, so only use it on a card you can erase.
3. Check the reported type: good, damaged (serious failure), limbo (the most common fake) and the rare wraparound and chain.
4. Compare “Usable size” (real capacity) with “Announced size” (advertised).

What if the card is fake?

First, ask the seller for a refund or file a complaint, with screenshots of the test. If you still want to use the card, F3 includes f3fix, which creates a partition covering only the usable area, for example sudo f3fix --last-sec=16477878 /dev/sdb. Then format the new partition and validate it again with f3write and f3read. If corrupted sectors remain, the card is probably defective and not worth the risk for important data. For photos you can’t lose, buy a name-brand card from a trusted retailer and keep a second copy elsewhere. If space is the issue, see how to free up space on Windows with the DISM command.

Frequently asked questions

Does the test erase what’s on the card?

H2testw and f3write write to all free space, and if the card is fake, old files can be lost. Back up first and preferably test an empty card.

How long does the test take?

It depends on size and speed. A full test writes and reads the entire card, so it can take hours on large cards. Quick tests such as FakeFlashTest’s and f3probe are much shorter.

Can I test the card inside my camera or phone?

That’s not recommended. Use a reader or adapter on a computer, which lets you choose the right drive and gives more reliable results.

Does a fast card mean a genuine card?

No. Speed and real capacity are different measures. A fake card can feel fast and still hold only a fraction of the advertised space.