Flash drive failed a capacity test? What to do

Your flash drive says it has plenty of space, but a capacity check reports errors or a smaller usable size. That can point to counterfeit storage, a damaged drive, or a test that did not finish correctly.

The Fake USB & SD Check writes test data to a drive and reads it back to check how much storage works. Here is how to run the check carefully, understand the result, and decide what to do next.

Check the drive safely

1. Back up any files you need before testing. The check writes data to the selected drive, so do not use it on the only copy of important files.
2. On a computer, open the Fake USB & SD Check in Chrome or Edge. The tool needs browser support for access to removable storage, so other browsers may offer reduced support or none.
3. Connect the drive directly to the computer if possible. In the browser's drive picker, select the drive you intend to test. Check its name and reported size carefully, especially if several removable drives are connected.
4. Run the check and let it finish writing and reading test data. Do not unplug the drive, close the tab, or let the computer go to sleep during the test.
5. Note the amount of storage that passed and any errors or interruption messages. If the test did not complete, treat that as an inconclusive result rather than proof of the drive's real capacity.

What the result means

The tested capacity is close to the drive's advertised size

The drive passed this capacity check. That does not prove it will never fail, but it suggests that the storage tested can be written and read back. If files still disappear or fail to open, copy them elsewhere and check the drive for errors before trusting it with important data.

The usable capacity is much smaller than advertised

This is a strong warning sign. Some counterfeit drives report a larger size to the computer than their physical storage can hold. Once writes reach the real limit, new data may overwrite older data or fail to read back correctly. Do not keep using the drive for files you need.

A difference between the label and the number shown by the operating system is not automatically fraud. Manufacturers usually state capacity in decimal units, while some operating systems display it using binary units. Formatting also uses a small amount of space. A large gap, especially when paired with read-back errors, is more concerning than that normal difference.

The test reports errors or stops at a repeatable point

A repeatable failure at roughly the same amount of data can mean the drive has less real storage than it claims. Errors at varying points can also come from a failing drive, a poor USB connection, or a problem with the computer. Try a different direct port, then test once more after protecting your files.

The test stops early or cannot access the drive

This does not establish the drive's capacity. The wrong drive may have been selected, the browser may not have access, or the connection may have dropped. Check the selected device and browser support before drawing a conclusion.

What to do after a failed test

1. Stop saving new files to it. A failed read-back check means the drive cannot be trusted to store everything it claims. Copy any readable files to another drive or computer, and verify that the copies open.
2. Confirm that you tested the right device. Disconnect other USB drives and memory cards, reconnect the one in question, and check its name and size in the drive picker. A result from a different drive will not help diagnose this one.
3. Try a different USB port. Connect the drive directly to the computer rather than through a hub or dock. If you use an adapter for an SD card, reseat the card and adapter before testing again.
4. Check whether the result repeats. After backing up the drive, run the check again on a computer using Chrome or Edge. If the test fails at a similar capacity again, the result is more persuasive than one interrupted run. Do not run repeated tests if the drive is getting hot, disconnecting, or making files inaccessible.
5. Reformat only after copying your files. Formatting can fix some file system problems, but it cannot add physical storage or repair a drive that lies about its capacity. In Windows 11, open File Explorer › This PC, right-click the drive, and choose Format. On macOS, open Disk Utility, select the drive's volume, and choose Erase. Formatting erases data on the selected volume, so check the selection and backup first.
6. Return or replace a drive that fails again. If it is new, contact the seller or manufacturer and provide the test result and purchase details. For steps to preserve files before replacing a suspect drive, see what to do about fake drive data recovery and when a USB drive needs replacement.

When the drive is probably faulty

If the capacity test fails on more than one computer, especially at a similar point, assume the drive is unreliable. A drive that disconnects, becomes read-only, or produces corrupted files also needs to be taken out of service, even if a later test passes. If the files matter and will not copy normally, avoid formatting or running more write tests. Those actions can make recovery harder.

For a recently purchased drive, request a refund or replacement rather than trying to repair its reported capacity. If it contains important files, save what you can to a separate device before returning it, and do not send the only copy of sensitive data with the drive.