How to spot an SD card that overwrites old data

You copy photos to an SD card, then find that older files have changed, disappeared, or stopped opening. One cause is a fake-capacity card: it reports more storage than it really has, so new data can overwrite data already saved.

Check the card before you rely on it. The Fake USB & SD Check fills the card with test data and reads it back to find its usable capacity. Back up anything important first, because the test writes to the card.

Check the SD card's real capacity

1. Copy your files somewhere safe. Copy, rather than move, any photos or documents you need. Do not run a capacity test on the only copy of important files.
2. Use a computer and a card reader. Open the Fake USB & SD Check in Chrome or Edge on a computer. Other browsers may provide reduced support or none.
3. Choose the SD card carefully. Follow the browser prompts to give the tool access and select the card. Check its name and size before continuing, especially if other removable drives are connected.
4. Let the test write and read back data. Keep the card connected and wait for the check to finish. Do not remove it or use it to save other files during the test.

A large card can take a while to check because the test needs to write and verify data across its capacity. A partial run does not confirm the whole card is genuine.

What the result means

The test verifies the card's reported capacity

The card read back the test data across the capacity it reported. That is a good sign, but it does not prove the card will never fail or that every old file is intact. Keep a second copy of anything you cannot replace.

The verified capacity is much smaller than the card's label

The card may be fake or damaged. A fake card can claim, for example, that it holds 128 GB while having much less physical storage. When writing passes the real limit, the controller may send new data to storage already used by older files. That can make older photos unreadable or replace them with newer data.

A test failure alone cannot tell you whether the card is counterfeit or failing. Either way, do not trust it with important files. Try another reader only after backing up what you can, then test again to see whether the problem follows the card.

The computer shows less space than the package

A modest difference is normal. Storage makers use decimal units, while computers may display a binary equivalent. For example, a card sold as 128 GB may appear as about 119 GiB, sometimes labeled as GB. That difference by itself does not mean the card is fake.

The test cannot access the card or stops early

Check that the card is connected, unlocked, and selected correctly. The full Fake USB & SD Check works in Chrome and Edge on a computer. A browser limitation, card-reader problem, or write-protected card can prevent a complete test.

What to do if files are being overwritten

1. Stop saving new files to the card. Continued writing can replace more data. If your card is in an SD adapter with a lock switch, slide it to the locked position while you decide what to do. The switch is a request to prevent writing, not a guaranteed safeguard.
2. Copy any readable files to another device. Use a different drive as the destination and check that the copied files open. If the files are valuable and already damaged, avoid experimenting with the card and ask a data recovery professional about options.
3. Check the reader and connection. Remove and reinsert the card, or try a different card reader or USB port. On Windows 11, open File Explorer › This PC to see whether the card appears. On macOS, check Finder › Locations. Do not format the card just to make it appear.
4. Run the capacity check again only after backing up. Test the card in a different reader if you can. If it reports a much smaller capacity or fails read-back in both readers, stop using it for storage.
5. Format only after recovering what you need. Formatting erases files. It may fix some file-system problems, but it cannot add missing physical capacity or repair failing memory. In Windows 11, use File Explorer › This PC, right-click the card, and choose Format. On macOS, open Disk Utility, choose View › Show All Devices, select the card, then choose Erase.

When the card needs replacement

If the test keeps finding a capacity mismatch or read errors after you have ruled out the reader, treat the card as unreliable. Do not use it for a camera, phone, or backup, even if it still appears to work for small files. Contact the seller or manufacturer if it is under warranty. For a low-cost card, replacement is usually safer than trying to repair it.

Why a fake card can look normal at first

A computer usually displays the capacity the card reports, not a measurement of its physical memory. A fake card can therefore look correctly sized in a file browser and may save a few small files without trouble. The problem may only appear after enough data has been written to pass the card's real limit.

The capacity check writes and verifies test data in your browser. Keep another copy of anything you want to preserve, and do not use a card that fails the check for important storage.