How to test a USB drive without risking files

You want to find out whether a USB drive has the capacity printed on its label, but it already contains documents you need. A full capacity check is not a read-only inspection: it writes data across the drive and reads it back.

To avoid risking important files, make and verify a separate backup before you test. If you cannot move or back up the documents, do not run a full capacity test on that drive. The Fake USB & SD Check can help identify a drive that reports more space than it can really store, but it needs to fill the selected drive to do that.

Prepare the drive for a safe test

1. Open Fake USB & SD Check in Chrome or Edge on a computer. The tool checks a USB drive or SD card by filling it and reading the data back.
2. Copy every file you want to keep to a different drive or another safe location. Do not treat a second folder on the same USB drive as a backup.
3. Open several files from the backup copy, including at least one important document. If the files do not open correctly, stop and make another copy before testing.
4. Reconnect the USB drive you plan to test and identify it by its name and capacity. If you have more than one removable drive connected, disconnect the ones you are not testing.
5. Follow the tool's on-screen instructions to select the drive and run the check. Keep it connected until the test finishes. Afterward, copy back only the files you need, and confirm they open.

If the documents are too valuable to risk, keep them off the drive and test an empty drive instead. For more preparation advice, see how to back up data before testing a flash drive.

What the result means

The reported capacity is confirmed

The drive stored and returned the test data across the space the check examined. That is useful evidence that its capacity is real, but it does not prove the drive will never fail or that every file is safe. Keep a separate backup of important documents.

The usable capacity is smaller than the label claims

The drive may be counterfeit, damaged, or otherwise unable to store data across its reported size. Do not put important files on it. If you just bought it, save the test result and contact the seller about a return or refund.

The test stops or reports read or write errors

The check could not complete reliably. The cause could be the drive, its connector, a card reader, a USB hub, or an interruption during testing. Do not rely on the drive for important files. Try again only after checking the connection and backing up anything on it.

How to protect your files

1. Keep the only copy somewhere else. Copy documents to a different physical drive or a trusted backup location before testing. If a file is essential, verify the backup by opening it from its new location.
2. Check which drive you selected. Compare the drive name and size shown by the computer with the removable drive you intend to test. If you are unsure, cancel instead of guessing.
3. Do not remove the drive mid-test. Wait for the check to finish before disconnecting it. An interruption can leave test data on the drive or an incomplete result.
4. Eject the drive before unplugging it. In Windows 11, use the removable drive's Eject option in File Explorer, or choose it from the taskbar's USB device menu. On macOS, click the drive's eject icon in Finder and wait for it to disappear before unplugging it.
5. Restore files from the backup. Once the check is complete and the drive is ejected safely, copy back the documents you need. Open a few files from the USB drive to confirm the copy worked.

Do not use a speed check as a substitute for a capacity check. A drive can transfer a small file quickly and still have less usable space than its label claims.

Why a capacity test can overwrite data

A fake drive can tell the computer that it has more storage than it physically contains. When writing data goes past the real capacity, the drive may overwrite earlier data instead of saving the new data in a separate space. That can damage files already stored on it, sometimes without an obvious warning.

A full capacity check writes across the space the drive reports, which is how it can reveal this problem. It is not designed to preserve documents already on the drive, and a successful result is not a substitute for a backup. Use it on an empty drive or one whose contents you have safely copied elsewhere.

The capacity check runs in your browser, but it writes test data to the removable drive you select. Keep valuable documents backed up on a different device.