Why counterfeit SD cards still format successfully

You formatted an SD card, and the computer reported success. That can feel like proof that the card is genuine, but it is not. A counterfeit card can accept a format while its real storage is much smaller than the capacity printed on the label.

Formatting prepares a file system for storing files. It does not necessarily fill the card with data and read everything back. To check the card's usable capacity, use the Fake USB & SD Check, which writes data to the drive and reads it back.

Check the SD card's real capacity

1. Open the Fake USB & SD Check in Chrome or Edge on a computer. This tool needs browser support for accessing external storage, so other browsers may offer a reduced version or none.
2. Back up any files on the card, or use a blank card. The test writes data to the card, so do not run it on storage that holds the only copy of important files.
3. Connect the card with a reliable reader. Choose the SD card when prompted, and make sure you have selected the right drive, not your computer's internal storage.
4. Run the test and leave the card connected while it writes and reads data. Wait for the check to finish, then compare the verified capacity with the card's advertised size.

A card labeled 128 GB may appear as about 119 GiB in a computer. Manufacturers usually label capacity in decimal gigabytes, while operating systems may display a binary unit called gibibytes. That difference is normal. A large gap beyond that, or a read-back failure, deserves further investigation.

What the result means

The test reads back close to the advertised capacity

The card appears able to store and retrieve data across roughly its claimed capacity. This is more meaningful than a successful format, but it does not prove the card's brand, age, quality, or long-term reliability. Keep another copy of important files and check the card again if it starts behaving strangely.

The verified capacity is much smaller than the label

The card may be counterfeit, damaged, or reporting a false size through its controller. Some counterfeit cards tell the computer they have more space than they really do. When the real storage fills, later writes may overwrite earlier files or fail, even though the computer still displays the larger capacity.

The test reports read-back errors or stops unexpectedly

The data written to the card could not be read back consistently. The card may be faulty, or a poor reader, adapter, or loose connection may have interrupted the test. If the card contains files you need, stop using it and copy what you can before doing more checks.

The card formats but files still fail later

Formatting only shows that the computer could prepare a file system using the capacity the card reported. It does not guarantee that every part of that reported space works. A card can therefore format normally and still lose files when it is filled beyond its true capacity.

What to do if the card fails

1. Protect any files first. Copy readable files to another drive. Avoid adding new photos or videos to a card that may be misreporting its capacity, because new writes can replace older data.
2. Check the reader and connection. Remove and reinsert the card, or test it in a different known-good reader. If the result changes, the reader, adapter, or connection may be involved. Do not remove the card while a test or file transfer is running.
3. Repeat the capacity check once. Use the Fake USB & SD Check again with the card connected reliably. A similar shortfall or read-back error points more strongly to a bad or counterfeit card than to a one-time connection issue.
4. Replace or return a card that fails again. Keep the receipt and packaging if you plan to request a refund or report a counterfeit. Do not trust the card with important files just because it can be formatted.
5. Format only after backing up and checking the card. Formatting erases the card's file structure and is not a capacity test. In Windows 11, open File Explorer › This PC, right-click the card, and choose Format. On a Mac, open Disk Utility, select the card, and choose Erase. Confirm the selected drive carefully before proceeding.

Why the computer accepts a false capacity

An SD card's controller communicates its available storage to the computer. The computer generally works with the capacity the controller reports, rather than independently measuring every physical memory cell. A counterfeit card can therefore announce a plausible size and allow the operating system to create a file system across that apparent space.

A quick format usually creates or refreshes file system information without writing test data across the whole card. Some full-format options write more data, but a format completing successfully still is not the same as a dedicated capacity check that writes data and reads it back. The read-back step is what helps reveal storage that is missing or does not retain what was written.

Use a blank card for a capacity test, and do not treat a successful format as proof that an SD card is genuine.