The dependable way to check a USB flash drive or SSD is a full-volume write-and-verify test. The test fills the available filesystem space with known files, reads those files back, and reports any data that could not be stored or verified. F3’s f3write and f3read implement this method. Back up the device, test it empty, and do not put important data on it until the complete test passes. See the F3 usage documentation for the tool’s documented workflow.
What an advertised-capacity test actually proves
The capacity printed on a package or shown by an operating system is a claim made by the drive’s controller and firmware. A device can report a large logical capacity even when its physical memory is smaller or unreliable. The only useful answer to “how much data can this drive really hold?” is to write data across the space and read it back.
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A small copy test is not enough. A counterfeit or failing device can accept files normally until writes reach the end of its genuine memory. At that point it may overwrite earlier files, return corrupted data, or report write errors. A full available-space test is designed to reach that boundary. It is more informative than copying a few gigabytes or checking the number printed in a properties window.
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Prepare the drive before you test it
Back up anything you need
Copy important files to another device before starting. A counterfeit drive can overwrite earlier contents when a test writes beyond the real memory limit. F3 specifically cautions against testing a nonempty card or drive because later test files can destroy files that appeared to be stored successfully. Treat the test as destructive to existing contents even when the selected method is described as a filesystem or “volume” test.
Empty and format the test volume
Delete files you do not need and make sure the test volume has as much free space as practical. A filesystem test can only fill free space, so a nearly full drive produces a partial result. If the device contains several partitions, select the mounted volume you intend to test and record its reported capacity and free space before starting.
Identify the correct device
Disconnect other removable drives where possible. Confirm the drive name and mount point in your file manager or operating system before entering a command. Selecting the wrong path can fill another disk, and a raw-device test can erase an entire disk. Never guess when two drives have similar names.
Plan for a long write and read
A complete test writes the available space once and reads it again. Large drives can therefore take many hours, depending on the flash, enclosure, USB connection, card reader, and computer. F3 notes that reader and port throughput can limit observed speed. Keep the computer powered, prevent sleep if it would disconnect the drive, and avoid copying other data to the same device while the test runs.
Use a stable connection
Connect the drive directly to a reliable port when possible. Do not unplug it during the write or read phase. If the result looks inconsistent and you suspect system caching, F3 recommends disconnecting and reconnecting before the read step, then running verification again. A connection problem can produce I/O errors, but reconnecting does not make a drive that fails at its true capacity trustworthy.
Choose the least-destructive method that answers your question
| Method | How it works | Data risk | When to use it |
|---|---|---|---|
| F3 filesystem test | f3write fills free space with numbered test files; f3read reads and verifies them. |
Existing files can be overwritten if the volume is not backed up and emptied first. | Best general-purpose method for a mounted USB volume or non-system SSD partition. |
| CapacityTester volume test | Performs a file-based write and verification test on a volume. | Use only after backing up and clearing the volume. | Useful when you prefer a graphical or Windows-oriented volume test. Its documentation is at the CapacityTester project page. |
| CapacityTester raw-disk test | Overwrites the device directly rather than testing files in a mounted volume. | Destructive: partitions must be unmounted and the device must be reformatted afterward. The documentation describes this test as somewhat experimental. | Only for a deliberately disposable device when a raw test is specifically required. |
| F3 probe | Probes capacity with substantially fewer writes than a full write/read test. | It modifies the device; follow the current tool instructions and assume data risk. | Useful when time or write volume is critical, but a probe is not a substitute for the broadest full verification when you need maximum confidence. |
The ordinary workflow should be a filesystem-based test. It checks the volume you will actually use and avoids unmounting and overwriting a disk at the raw-device level. A raw test is a separate operation, not a more “accurate” version that is safe to run casually.
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Run a complete F3 test on Linux or macOS
Install F3 using the package or build method appropriate for your operating system, then use the mounted path of the empty drive. The commands below illustrate the documented sequence; replace the example path with the path you verified on your own computer.
1. Start the write phase
f3write /media/your-user/DRIVEOn macOS, a mounted removable volume commonly appears under /Volumes:
f3write /Volumes/DRIVEF3 creates numbered test files until the available filesystem space is used. The final file can be smaller than 1 GB because the program works with whatever free space remains. Do not interpret that smaller final file as evidence that the drive has a smaller advertised capacity; it reflects the exact amount of free space available to the test.
2. Wait for the write report
Let the command finish without opening, moving, or deleting the generated files. Record the amount F3 says it wrote. This is the coverage of this run, not necessarily the nominal package capacity. If the drive was not empty, the reported amount may be much less than the device’s total size and cannot serve as a full-capacity result.
3. Reconnect if caching is a concern
When the operating system may still be caching writes, safely eject and reconnect the drive before verification. F3 documents this as a way to remove doubt about data that the system has not fully committed to the device. Do not skip the read phase merely because the write phase reported success.
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4. Run the read-and-verify phase
f3read /media/your-user/DRIVEOr, for a macOS mount:
f3read /Volumes/DRIVEF3 reads the numbered files and compares their contents with the known data pattern. Let it finish. A clean report means the tested files survived the round trip. Errors, unreadable files, changed data, or a verified amount smaller than the amount written mean the storage cannot be trusted for the tested capacity.
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5. Remove the test files only after verification
When the result is clean, delete the generated test files through the file manager or the normal cleanup process for your operating system, then eject the drive safely. If the result failed, preserve the report before deleting anything; it can help you document a return or warranty claim. Do not copy irreplaceable data onto the drive while you are still investigating.
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CapacityTester documentation describes a volume test that writes files and verifies them, which is the appropriate category for an ordinary Windows capacity check. Use the project’s documented instructions at https://github.com/hotnix87/h2testw, select the correct mounted volume, and make sure the volume is backed up and empty first.
H2testw is commonly associated with Windows write-and-verify testing, and readers often ask how to use it to detect a fake USB drive. However, a current authoritative distribution page was not established for this guide, so do not rely on an arbitrary download mirror. The important part is the method—known data written across the volume and read back—not a particular unofficial installer.
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CapacityTester also documents a raw-disk option. That option overwrites the device, requires partitions to be unmounted, and requires reformatting afterward; its documentation calls it somewhat experimental. Do not select it when you merely want to check a mounted drive containing files. If you choose a raw test for a disposable device, verify the device identity twice and assume every byte on it will be erased.
How the procedure applies to SSDs
The central principle applies to SSD storage as well: write known data across the capacity and verify it by reading the data back. The safest procedure depends on whether the SSD is an internal system disk, an external USB SSD, or a removable non-system volume.
Internal system SSD
Do not fill the operating system’s active system volume with a casual test. The filesystem method needs free space and creates large temporary files; filling the system volume can interfere with normal operation. Test a separate, backed-up non-system partition instead, or use a procedure specifically designed for the drive’s environment and maintenance requirements.
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External USB SSD
Mount the external SSD as a non-system volume, back it up, clear it, and use a filesystem-based write/read test. The USB bridge, cable, and port can affect throughput and can also cause disconnect errors, so keep the connection stable and interpret connection-related failures separately from a capacity boundary. A raw-device command is not universally safe merely because the storage is an SSD.
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Removable or secondary SSD volume
If the volume is disposable and mounted normally, the F3 workflow is the least complicated approach. Record the tested amount and the filesystem used. A pass confirms the data path and media handled that run; it is not an endurance test and does not predict how many future write cycles the SSD will survive.
Understand decimal GB, binary GiB, and real failures
Capacity labels and operating-system displays may use different units. F3 explains that some tools treat 1 GB as 230 bytes, while industry capacity labels often use 1 GB as 109 bytes. A drive sold using decimal gigabytes can therefore appear smaller when a system reports a binary-style value. Filesystem formatting and reserved space can reduce the usable figure further.
Unit conversion explains a modest display difference; it does not explain corrupted read-back data, verification errors, or a test that succeeds only over a small fraction of the claimed space. Once known data fails to survive verification, the issue is storage reliability or the data path, not merely GB-versus-GiB notation.
| Observed result | What it establishes | What to do next |
|---|---|---|
| Full write and full read complete with no errors | The tested amount was stored and read correctly during this run. | Delete the test files, keep the report, and use the drive normally while maintaining ordinary backups. |
| Write stops, or read-back reports changed or missing data | The drive or its connection could not reliably verify the tested data. | Do not trust the device for important files. Check the connection once, repeat only if a clear connection problem existed, and otherwise seek replacement or return. |
| Only a small sample was tested | That sample worked; capacity beyond it remains unverified. | Run a test that reaches as much available space as practical and report the exact amount tested. |
| Displayed size is lower but verification is clean | The difference may reflect decimal/binary units, formatting, or reserved space. | Compare byte counts and usable free space rather than judging the drive by the label alone. |
| Errors occur after reconnecting or on one port only | The connection path may be unstable; the result is still not a clean capacity pass. | Use a known-good port and cable, repeat the complete test, and keep the failing report if errors persist. |
Troubleshoot a failed or confusing test
The test did not reach the advertised size
First check how much free space was available when the write started. A filesystem test cannot use occupied space, and the last test file is allowed to be smaller than 1 GB. Empty the volume and run again if the first run was partial. Record the tested byte count so a partial pass is not mistaken for full validation.
The read phase disagrees with the write phase
Reconnect the drive before reading if caching is plausible, then run the read phase again. Keep other disk activity off the device. If the same files fail repeatedly, the drive has not demonstrated reliable storage at that capacity. Do not “solve” the problem by reducing the test size unless you clearly label the result as partial.
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The drive disconnects or reports I/O errors
Check the USB port, cable, enclosure, and reader, and repeat with a stable connection. Throughput can vary with the port and reader, so a slow test is not automatically a capacity failure. Repeated disconnects or verification errors remain a reason not to trust the drive, even if a different connection happens to complete a test.
The test is taking too long
A full write followed by a full read necessarily moves a large amount of data. Faster probing tools such as F3’s probe mode use substantially fewer writes, but they modify the device and require careful adherence to the tool’s current instructions. Use a probe when its trade-off is acceptable; use the complete filesystem test when you need the strongest practical evidence for the entire available volume.
The drive passed once and failed later
A pass is a record of that run, not a lifetime guarantee. Preserve backups and watch for new read or write errors. Flash media can become unreliable after the test, and no capacity test replaces a backup strategy.
Document the result so it is useful
Save the tool’s final report or a screenshot and note:
- the drive’s advertised capacity and the operating system’s displayed capacity;
- the filesystem and mounted volume tested;
- the amount of free space and the amount actually written;
- whether the test covered the full available volume or only a sample;
- the write and read results, including any error locations or messages;
- the connection used, especially if a different port or reader changed the outcome.
This record distinguishes a complete verification from a quick sample and gives a seller or warranty provider concrete evidence without claiming more than the test measured.
What to do after a failure
Stop using the device for irreplaceable data. If it contains files that are still readable, copy them to known-good storage before repeating destructive tests. Keep the failed report and describe the tested amount accurately. A failed result is consistent with fake capacity, defective media, or another reliability problem; it does not, by itself, prove that a seller intentionally supplied a counterfeit. Request a replacement or refund through the appropriate channel rather than continuing to trust a drive that could silently overwrite earlier files.
If a replacement is supplied, test the new device from an empty state as well. Do not infer that a different displayed number alone proves the replacement is genuine: only a complete write-and-read verification establishes what that particular unit handled during the test.
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Back up the drive, empty it, and run a write-and-read verification across as much available space as practical. F3’s f3write followed by f3read is the clearest general workflow; a documented volume test provides the same file-based principle on systems where that is more convenient. Treat raw-disk tests as destructive, distinguish decimal/binary display differences from actual corruption, and interpret a pass as evidence for the tested run—not a guarantee of future reliability.
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