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To check an SSD’s real capacity, first read the size of the whole physical disk, not a partition or the free space shown by a file manager. Then, if the drive is suspicious, run a full write-and-read verification test. A device listing reports what the SSD claims; only successfully writing and reading data across the claimed space tests whether that capacity is usable.
What “real capacity” means
An SSD exposes several different storage layers. Confusing them is the most common reason a healthy drive appears smaller than advertised.
| Layer | What it represents | What it can tell you |
|---|---|---|
| Physical disk | The entire SSD device as reported by its controller | The device-level capacity presented to the operating system |
| Partition | A slice of the physical disk | How much space has been assigned to one partition |
| Formatted volume | A file system inside a partition or container | Space available to that file system after its structures are created |
| Free space | Unused space inside a formatted volume | How much data can be added now, not the drive’s total size |
Windows, macOS, and Linux normally display the capacity the device reports. That is useful for identifying disks and partitions, but it does not prove that every advertised byte can retain data. A counterfeit flash device can report a large size while physically storing much less; data written beyond the genuine limit may be lost or corrupt the file system.
“Real” therefore has two meanings. The first is the reported device size: the number returned by the SSD controller and shown by the operating system. The second is the verified usable size: the range that can be filled with data and read back correctly. A listing command answers the first question. Only a complete write-and-read test addresses the second.
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Why an advertised 1 TB SSD does not show 1 TB of file space
Manufacturers use decimal units: one gigabyte (GB) is 1,000,000,000 bytes and one terabyte (TB) is 1,000,000,000,000 bytes. Operating systems and utilities may display the same byte count using binary units such as GiB or TiB, and formatting consumes additional space.
For example, 1,000,000,000,000 bytes is about 931 GiB before partition and file-system overhead. A volume can therefore be smaller than the number printed on the box without being defective. Some capacity is also reserved for formatting, metadata, firmware functions, and other internal operations. Compare the advertised decimal byte count with the whole physical device, then account separately for partitions, formatting, and existing files.
Do not compare a manufacturer’s decimal TB directly with a file manager’s “TB” label unless you know which unit convention that display uses. The safest comparison is an exact byte count for the entire physical device. After that, explain the difference in stages: partition boundaries, a container or logical-volume layer, file-system structures, and files already stored. None of those reductions alone demonstrates counterfeit hardware.
Prepare safely before inspecting or testing
- Back up important files. A read-only listing is low risk, but a full-capacity test writes across the target and can damage data on a counterfeit device.
- Identify the target by model, serial number, connection, and size. Do not rely only on a disk number that can change after reconnecting drives.
- Disconnect unrelated removable drives when possible. Fewer devices make selection mistakes less likely.
- Use read-only commands for inspection. Do not run commands such as clean, format, partition, or erase when you only need capacity information.
- Plan for a long test. A fill-and-verify pass writes and reads the tested space; large SSDs can take hours depending on interface and speed.
- Close applications using the target. Open files and background synchronization can change free space while a file-based test is running and make its result harder to interpret.
Testing an in-use system disk is especially inconvenient because the operating system, page files, and applications occupy space. Prefer an empty secondary disk or a disposable test volume. If the only available target contains valuable data, stop after the read-only inspection instead of accepting the risk of a destructive capacity test.
Check the reported capacity in Windows
Use DiskPart’s physical-disk listing
Microsoft documents DiskPart’s list disk command for viewing available disks. It shows whole disks rather than the free space inside a particular volume.
- Open Windows Terminal (Admin) or Command Prompt (Admin).
- Enter
diskpart. - At the DiskPart prompt, enter
list disk. - Match the SSD using its displayed number and size. If you need more detail, enter
select disk N, replacingNwith the correct number, and then enterdetail disk. - Enter
exitwhen finished.
DiskPart commands operate on the object that currently has focus. Selecting the wrong disk can make a later command destructive, so do not run clean, formatting, partitioning, or other modifying commands during an inspection.
The size in list disk is the capacity Windows has been told about. It is not a physical-authenticity test. A suspicious drive can appear normal here and still fail when its full addressable range is written and read.
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Use Fsutil for volume and NTFS details
fsutil is useful when you need file-system information, but it answers a different question from DiskPart. For an NTFS volume, run:
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fsutil fsinfo ntfsinfo C:Replace C: with the volume letter you are examining. NTFS information can include sector counts, total clusters, and free clusters. Those values describe the selected volume and its file system; they are not independent proof of the SSD’s physical capacity. A smaller NTFS volume may simply occupy one partition on a larger disk.
Use fsutil to reconcile a volume-level mystery—for example, why an NTFS volume has fewer free clusters than expected—not to certify the flash chips. Check the corresponding physical disk in DiskPart first, then treat the NTFS output as a description of the file system built on that disk.
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Run a full write-and-read test on Windows
For a suspect drive, a capacity test must cover the claimed space. The H2testw project describes a workflow that fills storage with test data and verifies the data afterward.
- Use an empty target or copy all important files elsewhere.
- Open H2testw and select the correct drive or test location. Verify the drive letter carefully.
- Choose the option to use all available space, or specify the range you intend to test.
- Start the write phase and allow it to complete.
- Run the verify phase and save the report, including the amount successfully written and read and any corrupted or missing data.
H2testw’s Windows version is described by its project README as experimental and may not estimate the result accurately in every case. Treat a failure as a serious warning, not as a precise measurement of the exact physical size. The test also writes across the target; if a fake device wraps around after its genuine capacity, later writes can overwrite earlier data and damage the file system.
A partial test has a narrower meaning. If you test only 200 GB of a purported 2 TB disk, you have shown that the selected 200 GB range worked under those conditions, not that the remaining 1.8 TB is genuine. For a meaningful authenticity check, select the entire claimed range and let both writing and verification finish.
Check the reported capacity in Linux
List whole disks and partitions with lsblk
Linux’s lsblk lists block devices and can show selected columns, topology information, JSON, or raw output. To inspect sizes in bytes and distinguish disks from partitions, run:
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For machine-readable output, use:
lsblk --json -b -o NAME,PATH,SIZE,TYPE,MODEL,SERIAL,TRAN,MOUNTPOINTSRaw or JSON output is useful for scripts, but it does not change what is being measured. lsblk reports block-device information; it does not validate that a suspiciously large device can physically store that amount.
Interpret Linux output correctly
- Compare the
diskrow with the manufacturer’s decimal byte capacity, not with a single mounted directory’s free space. - Check whether the mounted file system is a partition, a logical volume, or another layer inside the disk.
- Use the model, serial number, and transport columns to avoid testing the wrong device.
- Do not write a destructive test to a device node merely because its reported size looks plausible.
- Use
-bwhen comparing exact values; otherwise, rounded human-readable units can obscure small differences.
For scripts, JSON or raw output avoids parsing a display intended for people. It is still only an inventory of what the kernel was told. A script that records a 2,000,000,000,000-byte disk has not demonstrated that all those addresses retain data.
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Verify usable capacity on Linux
A genuine full-capacity check requires a utility that writes test data throughout the selected device or an empty file system and then reads it back. Choose a tool that explicitly supports your Linux file system and target type, follow its documentation, and test only a backed-up or disposable target. The essential result is the amount of data that can be written and read back without corruption; lsblk alone cannot provide that result.
Decide whether the utility should test a mounted file system by creating test files or address the raw device. A file-based test is less likely to overwrite partition metadata but can test only space available to that file system. A raw-device test can cover the complete device, yet selecting the wrong device can destroy every partition. Whichever mode you choose, verify the target identity twice and remove the test data or recreate the layout only after the report is saved.
Check the reported capacity in macOS
Inspect device size and block size with diskutil
macOS’s diskutil info -all reports information for attached disks and volumes. In the output, identify the physical device rather than interpreting only a volume entry. Pay attention to the device node, disk size, protocol, and block-size fields. The block-size information helps determine whether the device uses 512-byte or 4K blocks.
diskutil info -allmacOS can show several entries for one physical SSD: the device, partitions, containers, and volumes. A volume’s size is not automatically the size of the underlying SSD.
Record the physical device identifier and compare its byte-oriented size with the advertised decimal capacity. The block size explains how the device organizes sectors; it does not tell you whether every advertised sector is backed by reliable flash. A 512-byte or 4K report is therefore supporting detail, not a capacity verdict.
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What diskutil can and cannot prove
diskutil info -all is suitable for examining the size and block layout macOS reports. It is not, by itself, a full-capacity authenticity test. A counterfeit device can return a plausible size at the device level, so a suspicious SSD still needs a complete write-and-read verification with a trusted utility that supports macOS.
No single command in this inspection step establishes that every advertised byte is physically usable. Back up the target, select the physical device or an intentionally empty test volume, and use a compatible full-capacity verifier. Stop immediately if the utility identifies corruption or unexpected overwriting.
Do not treat a smaller APFS volume, container, or mounted volume as evidence that the SSD is undersized until you have located the physical-disk entry. macOS’s layered presentation can make several sizes appear for one device, just as partitions and volumes do on Windows and Linux.
Reported size versus verified usable size
| Question | Best evidence | Limitation |
|---|---|---|
| What size does the controller report? | Windows DiskPart, Linux lsblk, or macOS diskutil info -all |
A reported value can be false on counterfeit hardware |
| How much space is assigned to a partition or volume? | Partition and file-system details from the operating system | It may be only part of the physical disk |
| How much free space remains? | File manager or file-system tools | Existing files and overhead reduce the value |
| Can the claimed range store data reliably? | Full write-and-read verification | It takes time, consumes space, and can overwrite data |
These measurements answer different questions and should not be substituted for one another. A large physical-disk value with a smaller volume is normal after partitioning. A matching physical-disk value with failed readback is not reassuring: the controller may be presenting an address range that the underlying flash cannot retain.
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How to perform a safe full-capacity verification
- Choose a disposable target. Do not test a disk containing the only copy of your files.
- Confirm identity twice. Match the operating-system device identifier with the physical SSD’s model, serial number, and connection.
- Use a verifier that fills and reads the selected range. A simple size listing is not enough.
- Allow both phases to finish. A write-only pass can miss readback corruption; verification is the important second phase.
- Record the result. Note the successfully verified bytes, errors, unreadable ranges, and the date.
- Repartition or reformat only after the test. Any such operation changes the layout and can destroy the test data.
The H2testw README specifically warns that a fake flash drive may advertise more capacity than it has and that data written beyond the genuine limit can be lost. It also warns that the file system may be damaged once writing exceeds the real capacity. Those warnings apply to the general method: a full test is not harmless merely because it is software-based.
Interface equipment matters too. A loose cable, failing enclosure, inadequate power connection, or unstable port can produce input/output errors unrelated to the NAND capacity. If errors appear, check the connection and repeat only with a backed-up target. A repeatable failure at the same address is more concerning than a one-off interruption, but neither result justifies risking irreplaceable files.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to interpret common results
The volume is smaller, but the physical disk is the expected size
This usually indicates partitioning, formatting overhead, a container, or reserved space rather than a fake SSD. Compare the whole-disk entry with the volume entry before taking corrective action.
The operating system reports the advertised size, but verification fails near the end
This is consistent with a counterfeit or defective device whose controller reports a larger address range than the flash can retain. Stop using it for important data, preserve the test report, and replace or return the drive through the seller or manufacturer. Do not trust files that were written after the first corruption point.
The test reports less space than expected without corruption
Check whether the utility tested an existing partition or free space instead of the whole device. Also check decimal-versus-binary units and whether the test intentionally reserved space. A test that did not cover the claimed physical range cannot establish full capacity.
The test stops with input/output errors
Verify the cable, enclosure, adapter, port, and power connection, then repeat only on a backed-up target. A connection problem can interrupt a legitimate drive, while repeated errors at consistent addresses suggest a device fault. Do not erase or repeatedly rewrite irreplaceable data while diagnosing it.
The reported size changes after reconnecting
Record the model, serial number, operating system, and connection each time. A changing identity or capacity can indicate an unstable enclosure, firmware problem, counterfeit controller, or failing hardware. Use the physical device identity rather than assuming the same disk number refers to the same SSD.
The test succeeds, but the file manager still shows less space
This is normally a unit or layout explanation, not a contradiction. Recheck whether the test measured bytes while the file manager used GiB, and account for partitions, file-system metadata, reserved space, and files already present. Successful verification establishes the tested range under those conditions; it does not make every volume occupy the entire disk.
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A practical decision path
- Need only to see what is installed? Use DiskPart,
lsblk, ordiskutil info -alland inspect the whole-disk entry. - Is the displayed volume smaller? Compare the physical disk, partition, and volume layers and account for decimal units and formatting.
- Is the device newly purchased, unusually cheap, or inconsistent? Do not rely on the listing; perform a backed-up full write-and-read test.
- Did verification succeed across the claimed range? The tested range was usable under those conditions. Keep the report if the drive is important.
- Did verification fail or overwrite earlier data? Treat the SSD as unreliable, stop storing important files on it, and pursue replacement or return.
Key points to remember
- The whole physical disk, its partitions, formatted volumes, and free space are different measurements.
- Operating-system listings show reported capacity; they do not authenticate the flash memory.
- Manufacturer GB and TB are decimal units, while operating systems may display binary equivalents and reduce usable space through formatting.
- A full write-and-read test is the meaningful check for suspicious capacity, but it is slow, destructive to existing contents, and requires careful target selection.
- Back up first and avoid modifying DiskPart or other disk commands when you are only inspecting size.
Frequently Asked Questions
Can Windows, macOS, or Linux prove an SSD is genuine just by showing its size?
No. DiskPart, lsblk, and diskutil report the size supplied by the device controller. A counterfeit drive can report a plausible capacity, so genuine usability requires a full write-and-read verification.
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Why is a 1 TB SSD shown as roughly 931 GiB?
Manufacturers use decimal TB, equal to 1,000,000,000,000 bytes. Binary conversion produces about 931 GiB before partitions, file-system structures, reserved space, and existing data are considered.
Is checking free space enough?
No. Free space is the unused portion of one formatted volume. It can be reduced by files, formatting, or partitioning and says nothing about unassigned space or whether the underlying flash can retain all advertised bytes.
Is a full-capacity test safe on a drive containing files?
Not necessarily. The test writes across the selected range. On a counterfeit device, writes beyond genuine capacity can overwrite earlier data or damage the file system. Back up important files and use a disposable target.
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Clear out junk files and repair common Windows errorsFree Scan →Fix the driver behind crashes, sound loss and screen glitchesFind Drivers →What does fsutil ntfsinfo tell me?
It provides NTFS volume details such as sector counts, clusters, and free clusters. Those describe the selected file system, not independent proof of the physical SSD’s capacity.
What should I look for in lsblk output?
Use the row with TYPE=disk for the whole device and distinguish it from TYPE=part rows. Include model, serial, transport, and mount-point information so you do not test the wrong drive.
Does diskutil info -all verify all of a Mac SSD’s advertised space?
No. It is useful for reported size, device identity, and 512-byte or 4K block details, but a suspicious drive still needs a compatible full write-and-read test.
What if a test fails near the end of the drive?
Stop storing important data on it, preserve the report, and seek replacement or return. A controller that reports more space than the flash can retain is consistent with counterfeit or defective hardware.
The Bottom Line
Read the whole physical disk first, then separate reported capacity from partition, volume, and free-space figures. Decimal manufacturer units and formatting explain many normal differences. If authenticity is in doubt, back up the drive and run a complete write-and-read test on the correct disposable target; only verified readback shows that the claimed range is usable.
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