How do I verify the real capacity of a flash drive or memory card?
Test it with H2testw. The tool writes test files across the whole drive and reads them back, so it reports the capacity you can actually store data in — not the capacity the controller claims. A drive that reports 128GB but only holds 8GB will fail part-way through, and H2testw will tell you exactly where. The test is free, runs on Windows, and takes from a few minutes on a small drive to several hours on a large one. Do this on any filled-capacity order before you commit, whatever the supplier, because misrepresented capacity is one of the most common defects in the flash market.
Why a drive can report more capacity than it has
The reported capacity of a flash device is written into its controller firmware. Nothing physically forces that number to match the amount of working memory inside.
If a device is built with fewer or lower-grade memory dies than the label claims — or with dies that failed testing — the controller can still be programmed to report the full size. The drive appears normal in your operating system. It only fails when you write past the point where real storage ends.
That is the specific failure H2testw is designed to catch. It does not measure speed, endurance or longevity. It measures whether the capacity is real.
How to run the test
H2testw is a free Windows utility. The procedure is straightforward, and the only real cost is time.
- Empty the drive completely — the test fills it with test files, so do not run it on a drive that holds data you need.
- Download H2testw from the developer’s site and run it. It does not need installing.
- Select the target drive letter, leave the settings at default, and start the write-and-verify test.
- Wait. The test takes roughly 30–60 minutes per 64GB of claimed capacity, depending on drive and port speed.
- Read the result. It reports the data volume written without error and flags any sector that failed.
Reading the result
A pass means the drive stored and returned every test file correctly across its full claimed capacity. That is the minimum standard for a usable drive.
A failure — where writing stops early, or errors appear part-way through — means the usable capacity is smaller than advertised. The point at which it fails tells you roughly how much real memory is inside.
Note that any drive will show slightly less usable capacity than its nominal size even when healthy. That is not a defect: nominal capacity is quoted in decimal units while the file system works in binary, and the file system itself needs space for its own structures. A healthy 128GB drive typically shows around 119GB usable.
What to do if your drive fails
Do not keep it in service, and do not trust data on it. If you have already stored files past the failure point, recover what you can before writing anything further.
If the drive came from a supplier, raise it with them and test a sample from the rest of the batch. One failed unit in a batch is a warning about the whole batch.
If you are evaluating a supplier for volume production, test a sample from their production batch rather than a hand-picked sample unit. A sample unit shows you what the product can be; a batch sample shows you what it actually is.
Sources
Figures quoted on this page are from the published reporting listed above. Where a number is an estimate rather than a published figure, it is described as approximate.
Frequently asked questions
Is H2testw free?
Yes. It is a free utility distributed by its developer for Windows.
Why does the test take so long?
It writes the full capacity and then reads it all back to verify. That is roughly twice the drive capacity in data transfer, so a 128GB drive moves around 256GB of data. It is slow because it is thorough.
Is there a Mac or Linux alternative?
Yes. On macOS and Linux, the standard approach is to write test files to the drive, compute a checksum, read them back and compare. F3 (a command-line tool) does this on both platforms. The principle is identical: fill the drive with verifiable data and confirm every byte survives the round trip.
Does passing H2testw mean the drive is good quality?
No. It only confirms the capacity is real. It says nothing about write endurance, data retention over time, thermal behaviour or long-term reliability. Those depend on the silicon grade and controller inside the drive — which is why knowing the grade and source matters.
Should I test every unit in a bulk order?
That is rarely practical at full volume. The workable approach is to test a genuinely random sample from the production batch, then agree with the supplier what percentage of units are verified and how.
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