USB drive test for fake capacity and real speed

Three honest reasons. First, the test writes only to FREE space — existing files are never touched, and the report lists them separately ("Existing data — not tested"). Second, drive makers count in decimal gigabytes (128 GB = 128 billion bytes) while part of that is taken by the file system itself. Third, a few percent goes to formatting overhead. The report adds these up and shows the estimated total capacity, so you can see whether it matches the marketed size. For a 100% test, empty the drive first.

No. The tool writes sequentially and re-reads the oldest blocks as it goes. As soon as the drive starts losing data — returning zeros, garbage or another block's data — the test stops and shows up to where the drive keeps data. It does not happen instantly: while what was written still fits in the computer's memory, the computer answers from there and the fake stays hidden. In our measurements the verdict on a fake arrives about half an hour after the start on average, usually within 10 minutes to an hour.

To be 100% sure a drive is genuine, every byte has to actually be touched. There is no physically honest shortcut — any tool that claims "instant verification" is sampling at best and guessing at worst. Measured across real drives, expect roughly 2 minutes per GB: about an hour for 32 GB, 4–5 hours for 128 GB, and days for a multi-terabyte disk. That is why the test can be interrupted and continued: the data already written stays on the drive, and resuming picks up where it stopped instead of starting over.

Fake drives have a modified firmware that reports a larger capacity than the physical storage. A "128 GB" fake might only have 8 GB of real NAND. When you write beyond the real capacity, the controller either wraps around (overwriting earlier data), silently discards the writes, or returns zeros. Files appear to save but are corrupted when read back.

The principle is the same — write unique data and read it back. On top of that, this test re-reads old blocks while writing to catch a fake before the end, finds the exact boundary of the real capacity, and tells data read from the drive apart from data the computer served from its memory — for that part it asks you to replug the drive, as the F3 documentation recommends. Nothing to install.

No — the test writes only into free space and does not touch existing files; its own test files are removed when it finishes. The one caveat is the same H2testw gives: on a counterfeit or failing drive, data written past its real capacity can land on top of older data — a drive that writes in a circle overwrites its own beginning. So before testing a suspicious drive, copy off anything you need, and ideally test it empty, the day it arrives, rather than after a year of photos is on it.

The tool uses the File System Access API to write and read files directly on your drive. This API is currently only supported in Chromium-based browsers. Firefox and Safari do not support it yet.

Yes. The tool works with any storage device that appears as a folder in your file system — USB flash drives, SD cards, external HDDs/SSDs, and even network drives.

It means the system never mounted it, and there is nothing to test: this tool works on top of a file system, and at that moment there is none. Zero capacity in Properties, "insert a disk", a RAW file system and a missing drive letter all look alike, but their causes differ and so do their fixes. Start with a diagnosis by symptom — it works out the cause and gives you the commands for your system. If a repair brings the drive back, come here again and check how much of the memory is genuinely alive.