Check a USB drive's data integrity before resale

Before selling a USB drive or SD card, you want to know that it stores and returns data reliably, and that its capacity is real. Copying a few files is not enough: a counterfeit drive can appear to work until you write beyond its actual storage limit.

The Fake USB & SD Check fills a removable drive with test data and reads it back. It can reveal a fake capacity or read-back errors, but prepare the drive carefully because the test writes data to it.

Check the drive before resale

1. Back up any files you want to keep, then open a few from the backup to confirm they work. Do not run a full-capacity test on a drive that contains the only copy of important data.
2. Make sure the USB drive or SD card is empty, or that you have safely moved everything off it. The test writes data and uses the drive's available space.
3. On a computer, open the Fake USB & SD Check in Chrome or Edge. These browsers provide the file access needed for the full check. Other browsers may offer a reduced version or none.
4. Grant the browser access to the removable drive when asked, and select the drive you intend to check. Confirm its name and capacity before starting so you do not test a different disk by mistake.
5. Let the test finish writing and reading data. Do not unplug the drive, remove the card, or let the computer sleep during the check.

Use a stable connection. If possible, connect a USB drive directly to the computer rather than through a hub. For an SD card, use a card reader you trust. A loose connection or unreliable reader can interrupt the test and make the result difficult to interpret.

What the result means

The test completes and the read-back matches

The drive successfully stored and returned the test data across the tested space. This is useful evidence that the reported capacity is usable now. It is not a promise that the drive will never fail or that every file previously stored on it was intact.

The test finds less space than the label claims

The drive may be counterfeit, or the reader or connection may be interfering. Some capacity differences are normal because manufacturers and operating systems use different ways to display storage units. A large shortfall, especially if the drive stops accepting data well before its advertised capacity, is a warning sign.

The test reports read-back errors or stops partway through

The data returned did not match what the test wrote, or the drive became unavailable during the check. This can point to faulty or counterfeit storage, but a bad cable, card reader, USB port, or interrupted connection can also cause failure. Do not sell the drive as reliable until you have isolated the cause.

The browser cannot access the drive

This is not by itself proof that the drive is broken. The browser may not support the required file access, permission may have been denied, or the drive may not be mounted by the operating system. Try Chrome or Edge on a computer and reconnect the drive before deciding it has failed.

What to do if the test fails

1. Confirm the target drive. Check the drive name and capacity in the operating system before testing again. Disconnect other removable drives if you might confuse them.
2. Reconnect it directly. Try a different USB port, cable, or card reader. Avoid moving the drive while the test is running.
3. Check it in a supported browser. Use Chrome or Edge on a computer, then grant access to the correct removable drive when prompted. If access was blocked, change the browser's permission for the site and reload it.
4. Repeat the check through another reader or computer. If the same capacity mismatch or read-back error appears, the drive is more likely to be at fault than the first connection.
5. Do not rely on a failing drive. If it repeatedly fails, do not sell it as a working storage device. Replace it or clearly disclose the fault. Do not use formatting as a way to make a failing drive appear trustworthy.
6. Remove the test data before handoff. After the check, format the drive if you want to provide it empty. In Windows 11, open File Explorer › This PC, right-click the removable drive, and choose Format. On macOS, open Applications › Utilities › Disk Utility, select the correct external drive, and choose Erase. Confirm the drive name carefully because formatting erases its contents.

When the drive itself may be faulty

A repeatable read-back error across more than one reader is a strong reason not to trust the drive. Other warning signs include files disappearing, the drive disconnecting when untouched, or the computer reporting different capacities after reconnecting it. If the drive is under warranty, contact the seller or manufacturer. For inexpensive flash storage, replacement is usually safer than trying to repair it.

What a read-back test cannot prove

The test checks data that it writes and reads during the run. It does not inspect every part of the drive's electronics, predict how long the drive will last, recover files, or certify the health of its memory. A pass is a useful snapshot, not a guarantee for the buyer's future use.

Formatting also is not the same as securely erasing sensitive information. A normal format may remove file references without making old data unrecoverable. Flash drives and memory cards can remap storage internally, so repeatedly overwriting them is not a dependable sanitization method. If the drive held sensitive files, use a suitable manufacturer erase method or keep the media rather than passing it on.

The test runs on your device, and the test data stays on the removable drive unless you choose to move it elsewhere.