How to test a phone SD card for capacity errors

Your phone may show the card's full advertised size, but files can still disappear or become damaged when the card fills up. That can happen with a counterfeit card that reports more storage than it really has, or with a card that is failing.

The Fake USB & SD Check fills a card with test data and reads it back to check whether the reported capacity works. Run it on a computer with a card reader, not in a phone browser, and only after backing up anything you need.

Test the card safely

1. Open the Fake USB & SD Check on a computer in Chrome or Edge. It checks removable storage by writing and reading test data. Do not start the check if the card contains files you need.
2. Back up the card. Copy its files to another storage device or computer, then open a few of the copied files to make sure the backup is readable. A counterfeit card can overwrite data once a write passes its real capacity, so treat the check as destructive.
3. Check how the phone uses the card. On Android, look under Settings › Storage › SD card for whether it is portable storage or adopted as internal storage. Menu names vary by phone. Do not remove a card adopted as internal storage, since it may be encrypted and tied to that phone.
4. Eject a portable card before removing it. Use Settings › Storage › SD card › Eject on Android, if available. On Windows 11, open File Explorer › This PC, right-click the card, and choose Eject. On macOS, use Finder › Locations › Eject.
5. Connect it to the computer. Put the card in a compatible reader and select that card in the test. Follow the on-screen prompts to start the fill-and-readback check. Leave it connected until the check finishes, and do not choose another drive by mistake.

TooLab's Fake USB & SD Check works in Chrome and Edge on a computer. A phone browser may not be able to give the test direct access to a storage card, even when the card is connected through an adapter. If the card is adopted as internal storage, leave it in the phone and use another portable card for this kind of test.

What the result means

The test writes and reads the data successfully

The card passed a full-capacity write and readback check in this run. That is a useful sign that its reported capacity is usable, but it does not prove that the card will keep working indefinitely, that it is genuine, or that it meets a particular speed rating. Keep a separate backup of important photos and documents.

The test finds mismatched or unreadable data

The card did not reliably store and return all the test data. A counterfeit card may have less real storage than its label claims, causing later data to overwrite earlier data. A genuine card can also fail because of worn or damaged storage, a poor connection, or a faulty reader. Test it again with a different reader before deciding which is at fault.

The computer cannot write to the card

This can mean the card or adapter is locked, the file system is damaged, or the reader cannot access the card. Some full-size SD adapters have a small write-protect slider on the side. A phone connection alone may not provide the access the browser test needs.

The check stops before reaching the card's full size

Check whether the card is nearly full, whether another program is using it, and whether the reader or adapter is disconnecting. A result from a partial check cannot confirm that the entire advertised capacity is safe to use.

What to do if the card fails

1. Check the adapter and lock switch. If you use a full-size SD adapter for a microSD card, make sure its write-protect slider is not in the locked position. Reseat the card and reader firmly.
2. Try a direct connection. Connect the reader to another USB port instead of a hub, then check whether the computer can see the card. Avoid repeated plugging and unplugging while the test is writing.
3. Use another reader or computer. A failing reader can interrupt writes or reads and make a good card appear faulty. Repeat the check with a known-good reader, using Chrome or Edge on a computer.
4. Do not format it just to clear an error. Formatting erases the card and cannot make counterfeit capacity real. If the phone asks to format a card unexpectedly, cancel if you still need its files and make a copy or seek data-recovery help first.
5. Stop using a card that fails repeatedly. Copy any remaining readable files as soon as possible, but do not trust the card with anything you cannot replace. Contact the seller about a return or warranty claim, or replace the card.

When the card itself is the problem

If a failure follows the card across two readers or computers, while another card works in those same readers, the card is the likely cause. A counterfeit card and a genuine card with failing memory can produce similar symptoms. The test checks whether data can be written and read across the reported capacity, but it cannot prove a card's brand or manufacturing history. For a new card, save the test result and contact the seller before the return period ends. For a card that already holds important files, avoid further tests until you have copied what you can. See how to test an SD card for counterfeit storage for more checks before relying on a replacement.

Why capacity rollover damages files

Flash storage keeps track of data in physical memory locations. A counterfeit card can report a larger capacity than the memory it contains. Once software writes beyond that real limit, the card may loop back and overwrite data stored earlier. The phone can still display the advertised capacity, and files may look normal until you try to open them. A full write-and-readback test checks more of the card than copying one small file, but it still does not replace a backup.

Keep one copy of important phone photos somewhere other than the card, even if it passes the check.