How to test a new flash drive before trusting files

A new flash drive can look fine and still have less usable storage than its label claims. If its reported capacity is false, files may appear to copy successfully but become damaged or disappear when the drive fills up.

Test it before you trust it with photos, work, or backups. The Fake USB & SD Check fills the drive with test data and reads that data back to check its real capacity.

Test the new flash drive

1. On a computer, open the Fake USB & SD Check in Chrome or Edge. These browsers support the drive access this test needs. Other browsers may offer fewer features or none.
2. Use an empty drive, or copy anything on it somewhere safe first. The test writes data to the drive and uses its available space. A capacity test can put existing files at risk, especially if the drive is counterfeit.
3. Connect the flash drive directly to the computer if possible. Follow the tool's prompts to select the drive you want to check. Check the drive name and capacity carefully so you do not test another disk by mistake.
4. Start the test and let it finish. The tool writes test data and reads it back. Do not unplug the drive, close the browser tab, or let the computer go to sleep while the check is running.
5. Review the result before using the drive for important files. A complete read-back check is more useful than simply seeing the drive listed in File Explorer or Finder.

A large drive may take a while to test, depending on its speed and capacity. Keep the computer connected to power if the test is lengthy, and do not use the drive for other transfers during the check.

What the result means

The full test completes and the data reads back correctly

The drive passed this capacity check: the space tested accepted data and returned it correctly at the time of the test. That is a useful first check before copying important files. It does not prove the drive will never fail, or that it will keep data safe for years. Keep another copy of anything you cannot replace.

The test reports errors or less space than expected

The drive may have less real storage than advertised, or it may have a fault. Some counterfeit drives report a large capacity to the computer but overwrite earlier data when you store more than their actual capacity. Do not use a failing drive for important files. For more on why this happens, see why counterfeit storage works until the drive fills up.

The capacity looks a little lower than the package says

A small difference by itself is normal. Manufacturers use decimal units for the advertised capacity, while operating systems may display storage using a different unit convention. Formatting also uses some space for file system information. For example, a drive labeled 128 GB may appear near 119 GiB before that overhead. A much larger difference, or a read-back error, is a reason to investigate.

The test cannot access the drive or does not finish

This alone does not prove the drive is faulty. The browser may not support the required access, the wrong drive may have been selected, or the device may have disconnected. Try the checks below before deciding what to do.

How to fix a failed or incomplete check

1. Make sure you chose the right drive. Disconnect other removable drives if you are unsure which one is which. Use the drive's name and capacity to identify it before allowing the test to write data.
2. Use Chrome or Edge on a computer. The Fake USB & SD Check needs browser features that work in Chrome and Edge on desktop computers. If you opened it in another browser or on a phone, try again on a computer with Chrome or Edge.
3. Reconnect the drive. Safely eject it if the operating system still shows it as connected, then unplug and reconnect it. Try a different USB port, preferably one directly on the computer rather than a hub or dock.
4. Close other programs using the drive. A file manager window or another app copying files can interfere with the check. Finish or cancel those transfers, then run the test again on the empty drive.
5. Check whether the computer can read the drive. On Windows 11, open File Explorer and check This PC. On macOS, check the drive in Finder. If the drive does not appear there either, try another port or computer before concluding that the test itself is the issue.
6. Do not format the drive just to clear a failure. Formatting changes how the drive is organized, but it cannot create storage that is not physically there or repair a failing memory chip. If the same capacity or read-back error returns, stop using it for anything important.

When the drive may be faulty

If the test repeatedly reports errors, stops at a similar capacity, or reads back different data after reconnecting, treat the drive as unreliable. If it is new, ask the seller for a refund or replacement while you are within the return period. Keep the packaging and order details, but do not put private files on the drive as proof. See when a USB drive needs replacement, not repair for signs that it is time to stop using one.

A successful test is a useful screen, not a guarantee of future health. Keep a second copy of important files on a separate device or backup location.