Why a counterfeit drive can seem fast at first

A new USB drive copies the first few files quickly, so it seems genuine. Later, transfers slow to a crawl, files disappear, or the drive reports an error. A fast start does not prove the drive has the capacity or reliability printed on its label.

Short tests can miss both fake capacity and slowdowns that appear after a cache fills. Here’s why that happens and how to check a drive before trusting it with important files.

Check the drive safely

1. Back up anything on the drive that you want to keep. The Fake USB & SD Check fills the selected drive with test data and reads it back, so use an empty drive if possible.
2. Open the tool in Chrome or Edge on a computer. These browsers support the full version of the drive check. Connect the USB drive or card reader directly, and allow the browser to access the removable drive when asked.
3. Select the drive you want to check, then let the test write and read its test data. Keep the drive connected and avoid using it for other transfers while the test runs.
4. Compare the verified capacity with the amount the drive claims to hold. A small difference in the number shown by the operating system can come from how storage units are counted and space used by formatting. A large shortfall or failed read-back is not a normal unit conversion.

If you only want to compare transfer speeds, the USB & SD Speed Test measures read and write performance. A speed result alone cannot confirm that every part of a drive’s advertised capacity stores data correctly.

Why the first copy can look fast

Several ordinary features can make early transfers look better than later ones. Some flash drives use a fast temporary cache, a portion of the memory that accepts data quickly. Once that cache fills, the drive has to write directly to slower flash memory, and the transfer rate can fall sharply. The slowdown does not automatically mean the drive is fake.

Your computer can also use memory to buffer a copy. The progress window may show that the transfer has finished while the drive is still writing data in the background. Removing the drive as soon as the window closes can interrupt that work. Use the system’s safe eject option and wait until it says the device can be removed.

A counterfeit drive can exploit the same short-test pattern in a more serious way. Its controller may report a much larger capacity than the physical flash memory can store. A small file, or a short benchmark that writes only to the real memory, may complete normally. Once new data exceeds that space, the drive may overwrite earlier data, corrupt files, or return errors. The reported capacity can still look correct in your computer’s file manager.

What the result means

The full advertised capacity passes read-back

The test could write and read the amount checked, which is good evidence that the drive’s stated capacity is available. It does not guarantee the drive will stay reliable over time, or that it will maintain one speed during a long transfer. Keep another copy of important files.

The verified capacity is much smaller than the label

The drive may be counterfeit, damaged, or unable to address all its flash memory correctly. Stop using it for files you need. Save the test result and receipt if you plan to request a refund or report the problem to the seller.

The test finds errors or cannot finish

A read-back error means the data did not come back as written. A loose connection, failing drive, or faulty card reader can also cause a failed test. Reconnect the drive directly to another USB port and test again before deciding which part is at fault.

The drive passes capacity but slows down

A speed drop after a fast start can be normal when a temporary cache fills. It is a separate issue from fake capacity. If the slowdown is extreme or happens alongside errors, test the drive in a different port and check it again with no other transfers running.

What to do if the speed seems wrong

1. Test the drive without a hub. Connect it directly to the computer. A hub, extension cable, or card reader can limit the connection or cause interruptions.
2. Try another port and reader. If a memory card is involved, test it in a second reader. A different result can point to the original port or reader rather than the card.
3. Repeat the test on an empty drive. Close file transfers and other programs using the drive. Back up the drive first, because a capacity check writes a large amount of test data.
4. Verify capacity before trusting the drive. Use the Fake USB & SD Check, not just the number shown in File Explorer or Finder. If read-back fails or the verified space is far below the label, do not store your only copy of anything on it.
5. Return or replace a drive that fails twice. A known-good port and reader reduce the chance that the connection caused the error. If the drive still cannot verify its capacity, ask the seller for a refund or replace it with a drive from a reliable source.

Why a quick speed test is not enough

A speed test and a capacity test answer different questions. Speed describes how quickly a particular amount of data was read or written. Capacity verification checks whether data across the drive can be stored and read back without overwriting earlier data. A counterfeit drive can look fast during a small test and still fail when filled.

For more on what can happen when a fake drive runs out of real memory, see Why counterfeit storage works until the drive fills up. Until a new drive passes a full read-back check, do not use it as the only home for photos, work, or backups.