Check a replacement flash drive for fake capacity

You bought a replacement USB drive or SD card, but the label alone cannot prove it holds the amount of data it claims. Some counterfeit drives report a large capacity even though they contain much less flash storage. They may seem to work at first, then overwrite older files when you fill them.

Test the drive before putting photos, work files, or backups on it. TooLab's Fake USB & SD Check fills the selected drive and reads the data back to look for a capacity mismatch. Use it on a computer in Chrome or Edge, and make sure the drive is empty first.

Test the replacement drive

1. Back up anything on the drive, then make sure it is empty. The test writes data across the drive, and a counterfeit drive can overwrite earlier data when its real capacity is exceeded.
2. Connect the drive to your computer. If you are testing an SD card, insert it in a card reader. Open the Fake USB & SD Check in Chrome or Edge on a computer. Other browsers may offer limited access or none.
3. When the browser asks you to choose a drive, select the replacement card or USB drive. Check its name and capacity carefully so you do not test another storage device by mistake.
4. Run the full fill-and-read-back check and wait for it to finish. Do not unplug the drive or close the browser while it is working. A large or slow drive can take a while to test.

The check reads and writes data on the selected drive in your browser. It does not establish that the brand or seller is genuine, and a pass cannot guarantee that the drive will last indefinitely.

What the result means

The full check finishes without a capacity or read-back error

The tested space accepted data and returned it correctly. That is useful evidence that the replacement has the capacity it presents to the computer. It does not prove the printed brand, advertised speed, or long-term reliability. Keep a separate backup of important files even after a successful test.

The check finds less usable capacity than the drive claims

The device may be counterfeit, defective, or reporting its size incorrectly. Fake drives can claim, for example, that they hold hundreds of gigabytes while containing only a small fraction of that amount. Once writes pass the real storage limit, new data may replace earlier data instead of being saved safely.

The read-back check reports errors or the drive disconnects

Data did not come back as expected, but the result does not by itself prove counterfeiting. A damaged card, poor adapter, loose connection, or faulty USB port can also interrupt a test. Stop using the drive for important files and check the connection before deciding what to do.

The computer shows a slightly smaller number than the package

This is often normal. Drive makers use decimal units, while many operating systems display capacity using larger binary units. A drive sold as 128 GB may appear as about 119 GiB before formatting and file-system overhead. A small difference is not the same as a read-back test finding that the drive cannot hold its claimed data.

What to do if the replacement fails

1. Check the connection. If the drive disconnected or the test stopped unexpectedly, reconnect it directly to another USB port. For an SD card, reseat it in the reader or try a different reader. Avoid a hub for this check.
2. Run one careful retest. Make sure the drive is empty and that you selected the right device. If the same capacity or read-back error appears, do not keep repeating full-drive tests. Repeated writes are not a way to repair a counterfeit or failing drive.
3. Keep evidence for the seller. Note the test result and save the packaging, receipt, and order details. If you can, take a clear photo of the capacity printed on the product and packaging. Do not store important files on the drive while you wait for a response.
4. Ask for a refund or a verified replacement. Contact the seller promptly and explain that the drive failed a full-capacity read-back check. If this was already a replacement, include the earlier order or case information and request a refund or a different unit.
5. Eject the drive before removing it. On Windows 11, use the taskbar's safely remove hardware option and select the drive. On macOS, open Finder and use the eject icon beside the drive before disconnecting it.

Formatting the drive will not add missing flash storage or make a counterfeit capacity claim true. Do not try to fix a failed capacity check by reformatting and trusting the same drive again.

Why a drive can look fine until it is full

A computer can display the capacity a drive reports, not verify that every address maps to physical storage. A fake device may claim a large size while redirecting writes beyond its real limit to the same small area. Basic tasks such as opening the drive, copying one small file, or checking its properties can all appear normal.

A full write followed by a read-back check is more useful because it tests data across the claimed space. The card reader or adapter is part of the chain, though, so a connection fault can affect the result too. If a second test still fails after you have checked the connection, return the drive instead of relying on it for a backup or replacement storage.