Why checking free space cannot prove drive capacity
A drive can report plenty of free space and still have less real capacity. Learn why, and how a write-and-read test checks it.
Your USB drive says it has hundreds of gigabytes free, but files disappear or become corrupted when you copy more data to it. That free-space number is not proof that the drive can really store that much.
To check the usable capacity, data has to be written across the drive and read back. The Fake USB & SD Check does this on a USB drive or memory card, so you can see whether its storage works as reported.
Check the drive's real capacity
- Back up any files you need. The Fake USB & SD Check writes test data to the selected drive and may use its available space.
- Open the tool in Chrome or Edge on a computer. Choose the USB drive or SD card when the browser asks you to select a location.
- Start the capacity check and leave the drive connected. The tool writes data and reads it back, which can take a while on a large or slow drive.
- When the check finishes, compare the amount of data verified with the capacity you expected. Do not remove the drive while the test is running.
If you are not sure which drive is which, check its name and capacity in your computer first. In Windows 11, open File Explorer › This PC. On macOS, select the drive in Finder and choose File › Get Info. Be careful to select the removable drive, not your computer's internal storage.
What the result means
The test writes and reads the expected capacity
The data the test checked could be stored and read back. This is much stronger evidence than a free-space figure alone. It does not guarantee the drive will last forever or perform at its advertised speed, but it suggests the tested capacity is usable now.
The check reports errors or verifies less than expected
Some data could not be read back correctly, or the drive could not provide the expected amount of working space. A counterfeit drive may claim a larger size than its memory chips contain. When that happens, new data can overwrite older data without an obvious warning, so stop using the drive for important files.
The test cannot access the drive or stops early
This does not by itself prove that the drive is fake. The browser may not have access to the selected location, the drive may be write-protected, or a connection may have dropped. Check the fixes below, then run the test again before drawing a conclusion.
The computer shows less capacity than the label
A difference in the displayed number can be normal. Drive makers usually label capacity in decimal units, while some operating systems display it using binary units. A drive sold as 1 TB may appear to have about 931 GiB before accounting for formatting. That unit difference is not the same as a failed write-and-read check.
How to check a confusing result
- Confirm the selected drive. Check the drive name and reported capacity in File Explorer on Windows 11, or use Finder › File › Get Info on macOS. If you chose the wrong drive in the browser, select the removable one and repeat the check.
- Make a backup before testing again. Copy important files to another trusted location. A capacity test writes data, and a fake drive can overwrite existing files when its real memory fills up.
- Use a supported browser on a computer. The Fake USB & SD Check works in Chrome and Edge on computers. If you are using another browser, switch to one of these before troubleshooting the drive itself.
- Check write access and the connection. Make sure the drive is not locked by a physical write-protect switch. Reconnect it directly to another USB port, or reseat the memory card in its reader, then try again.
- Test the drive in another port or reader. A faulty adapter, card reader, hub, or loose connection can interrupt writing and reading. If possible, use a different reader or computer to see whether the same result follows the drive.
- Replace the drive if the failure repeats. If the test repeatedly finds errors or cannot verify the capacity on more than one connection, do not trust it with important data. Return it to the seller or manufacturer if it is under warranty, or replace it.
Why free space is only a reported number
Your operating system gets free-space information from the drive's file system, the system that tracks files and available areas. That figure describes what the drive reports as available. It does not make the computer fill every area with unique data and check that the same data comes back.
A counterfeit drive can be programmed to identify itself as a much larger device than it is. It may accept file copies at first, then overwrite earlier data when the small amount of real memory is full. The file list can still look normal, and the free-space display may not expose the problem.
A write-and-read check looks for that mismatch by filling storage with test data and checking what returns. Formatting a drive, viewing its properties, or copying a few small files does not substitute for that check. Keep the drive connected until the test has finished, and use a separate backup for anything you cannot afford to lose.
