A fake USB drive or SD card reports a larger capacity than its flash holds. Files written past the real size are lost or overwrite earlier ones, often without an error.
- The capacity in the properties window comes from the drive itself, so it says whatever the fake reports. Only writing data and reading it back can find where the real capacity ends.
- The USB drive test does that in Chrome or Edge: it writes numbered files of known data into a folder of its own, reads them back and checks every byte. The first file that fails marks the end of what the drive really keeps.
- A quick test checks only what it wrote. Ruling a fake out needs the full check, which fills the free space.
The short answer
Fill the drive with data you can check, then read it all back. A genuine drive returns every byte; a fake returns old or empty data past its real size. Native tools such as H2testw and F3 do this, and so does the USB drive test in the browser. The size printed on the box and in the operating system is not evidence either way.
We have no recorded runs of this test in our validation notes yet, so this guide quotes no measured results. Every line and constant below is the one the test's code applies.
How the test checks every byte
You pick a folder on the drive and the page makes its own folder inside it, named systemcheck-drive-test- and a number for the run. It writes files of 256 MiB, each built from 1 MiB blocks. Every block starts with its file number, its block number and the run's seed, and the rest is generated from them, so the expected content of every byte is known without storing a copy.
Reading back, a block that comes back blank, holds old data, or carries another block's header (which is how a drive that wraps around fails) is counted as bad. The first bad file tells you how far the drive kept your data. When the test ends, it removes only its own folder.
Quick test or full check
The quick test writes 256 MiB, 1 GiB and 4 GiB, your choice, and says what it checked: only that amount. Fake drives fail past their real size, so a quick pass does not rule one out. The full check keeps writing files until the drive reports it is full, then reads them all back. It takes as long as filling the drive takes, and it checks only free space, not space other files already use.
Speed, and the cache cliff
Write speed is timed per file, until the browser has committed it. When later files all run at under 50% of the early files' median and stay there, the test reports the point where that began: the shape of a drive's fast cache running out. The 50% is our line. Read speed can come from the system's memory cache until the drive is unplugged and plugged back in, so the test offers that step for a true read figure.
An SD card or microSD card
In a card reader the card is a drive like any other, with folders, so the same test works on it. Pick a folder on the card, run the full check, and read the result the same way. A card that loses data past a point should not hold anything you cannot lose.
Step by step
- 01
Open the USB drive test in Chrome or Edge
Plug in the drive or card and pick a folder on it.
- 02
Choose the full check
It writes until the drive is full, then reads every file back.
- 03
Read where it failed, if it did
The amount before the first bad file is what the drive really kept.
Terms used here
Questions
- What do I do if the drive fails partway?
- Do not store anything on it you cannot lose. Format it and run the full check again; if it fails at the same point, its real capacity ends there. Failing flash can lose data too, so a repeat at a different point is also a reason to stop using it.
- Why does the test need Chrome or Edge?
- It writes to a folder you pick through the File System Access API, which Chrome and Edge offer on a computer and Firefox and Safari do not.