Check read-back integrity on a high-capacity USB drive

A thumb drive can show a large capacity on its label and still fail when you try to use all that space. If it is counterfeit or faulty, files may become unreadable or overwrite other data once the drive fills up.

The Fake USB & SD Check writes test data to the drive and reads it back to check its usable capacity. This is a destructive kind of test in the sense that it uses storage space, so back up anything important first and use an empty drive if possible.

Test the drive's read-back integrity

1. Back up the drive's contents to another device. For the clearest result, use an empty drive with no files you need to keep. The check may use most or all available space while it tests.
2. On a computer, open the Fake USB & SD Check in Chrome or Edge. This tool works in those browsers on a computer. Other browsers may offer a reduced version or no access to the drive.
3. Connect the thumb drive, choose it in the browser's drive picker, and allow access when prompted. Check the drive name and capacity carefully so you do not test the wrong device.
4. Start the check and let it finish writing and reading data. Keep the drive connected. A high-capacity device can take a long time to test, especially if it has a slow write speed.
5. Review the result. Note the capacity the check was able to verify and whether the data read back correctly. Do not unplug the drive while the check is still running.

A quick format or a file copy is not a substitute for this check. A counterfeit drive can report its advertised size to the computer, even when it has much less physical storage inside.

What the result means

The check reads back data across the expected capacity

This is a good sign: the drive stored and returned the test data across the space the check was able to examine. It does not prove the drive will never fail, or verify files that were on it before the test. Keep a separate backup of anything important.

The verified capacity is far below the label

The drive may be counterfeit, damaged, or using a controller that reports more space than its flash memory can hold. A small difference between the label and the computer's displayed number can be normal. For example, a 1 TB drive may show about 931 GiB before accounting for formatting. A result that is a small fraction of the advertised size is not explained by that unit difference.

The check reports a read-back problem or stops early

The test data did not come back as expected, or the drive became unavailable before the check finished. A loose connection, a failing drive, or an interrupted test could cause this. An incomplete check does not confirm the full capacity, so do not rely on the drive for important files yet.

How to troubleshoot a failed or incomplete check

1. Confirm you selected the right drive. Disconnect other removable drives if it is hard to tell them apart, then reopen the picker and select the thumb drive you intend to test.
2. Use a supported browser on a computer. Run the check in Chrome or Edge, not on a phone or in a browser that does not support the drive access feature. If access was denied, choose the drive again and grant permission when asked.
3. Connect the drive directly. Remove a USB hub or extension cable for the next test, and try another port. Avoid moving the drive or computer while the check is running.
4. Make sure the computer stays on. A sleeping computer or a disconnected drive can interrupt a long test. Keep the computer awake and do not close its lid during the check.
5. Repeat only after protecting your files. If the first run stopped unexpectedly, back up anything you can still read, then try again on a stable connection. If the same capacity shortfall or read-back error appears, treat the result as a real warning. See what to do when a flash drive fails a capacity test.

When the drive may be faulty

If the same problem appears on a second port or computer, the thumb drive is likely counterfeit or failing. Do not store your only copy of photos, documents, or backups on it. If it is newly purchased, save the test result and ask the seller for a return or refund. If files on it matter and have become unreadable, stop writing to the drive and do not format it. Repeated tests can write more data, so they are not a safe first step for recovering files.

Why a read-back test finds fake capacity

Some drives use a controller that tells the computer the device is larger than its actual flash storage. The computer may accept writes at first, but when the real storage fills, later data can overwrite earlier data or fail to come back correctly. A read-back check compares what it wrote with what it can read from the drive, helping reveal that mismatch.

A successful check is evidence about the drive's tested storage, not a guarantee of its future reliability or brand authenticity. Keep an independent copy of any files you cannot afford to lose.