SD card fails near its advertised capacity

Your SD card works at first, then files fail to copy as it gets close to the advertised size. Sometimes earlier files disappear or become unreadable. That pattern can point to a card that reports more storage than it really has, but a faulty reader, damaged card, or file-size limit can cause similar trouble.

Back up anything you need before testing. The Fake USB & SD Check fills a card with test data and reads it back to find its usable capacity. It is designed to expose cards that fail near their claimed limit, but it is not a data recovery tool.

Check the card's real capacity safely

1. Back up the card, then make sure the copy opens. Test only a card that is empty or whose contents you can afford to lose. For more on preparing a drive, see how to back up data before testing a flash drive.
2. On a computer, open the Fake USB & SD Check. Its full drive access works in Chrome and Edge. Other browsers may offer a reduced version or none.
3. Connect the card using its usual reader, choose the card when the browser asks for access, and run the capacity check. The tool writes test data to the card and reads it back, so keep the computer awake and do not disconnect the card during the test.
4. When the check finishes, note the verified capacity and whether the read-back matched the data written. If it fails, repeat once with a different reader or adapter before deciding the card is bad.

A capacity test can take a while, especially on a large or slow card. Do not use the card for important photos or files until it passes a full write and read-back check.

What the result means

The check reads back less data than it wrote

The card did not reliably store all the test data. A counterfeit card may report a large capacity to the computer while holding much less. When the real storage fills, new data can overwrite older data or fail to read correctly. Stop trusting the card for files, even if it still appears in File Explorer or Finder.

The result is similar to the card's stated capacity

The card passed this capacity check in the reader you used. If copies still fail in a camera, phone, or game console, the problem may be that device's compatibility, its file system, or a damaged connection. A pass is useful evidence, but it cannot prove the card will never fail or that every device supports it.

The check fails in one reader but passes in another

The first reader, adapter, or connection may be unreliable. Try a different USB port and reader, and avoid hubs while testing. If the card passes consistently in one reader but not another, replace or troubleshoot the reader before blaming the card.

Only one very large file will not copy

This may be a file-system limit rather than fake capacity. FAT32, a common format on older cards and devices, cannot store a single file larger than 4 GB, even when the card has plenty of free space. Check the format and the device's supported formats before reformatting.

What to do after a failed check

1. Stop writing important files. Further use can overwrite data or make existing files harder to recover. If you need files that are already on the card, do not run more capacity tests. See what to do first when recovering data from a fake drive.
2. Repeat the test with a different reader. Use a direct computer port if possible, and check that the card is seated properly. A second failure near the same capacity makes a card fault more likely.
3. Check the card in the device that uses it. For a camera, phone, or handheld console, check the manufacturer's supported card type and maximum capacity. A device may reject a genuine card that exceeds its limits.
4. Do not format it as a quick fix. Formatting can remove access to files, and it cannot make a card's physical capacity larger. If the card has passed a full test and you have a backup, format it using the device that will use it, following that device's instructions.
5. Return or replace a card that fails again. Keep the receipt and packaging, and save a note of the test result. If it was sold as new, contact the seller or manufacturer rather than using it for anything important.

Why the number shown by your computer is smaller

A difference between the label and the computer's capacity is normal. Manufacturers use decimal units, where 1 GB is 1 billion bytes. Some operating systems display storage in binary units, so a card labeled 256 GB may appear close to 238 GiB. Formatting also uses a small amount of space. That ordinary difference does not explain a card that fails, loses files, or becomes unreadable after only part of its advertised capacity has been written.

A genuine card can also fail because it is worn, damaged, or counterfeit. A reader can cause errors too. The read-back check helps separate a real capacity problem from a misleading label, but one test cannot identify the exact cause of every failure.

The capacity check runs in your browser and writes test data to the selected card, so keep a verified backup somewhere else before you start.