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Usually, nothing is missing. Drive makers advertise capacity in decimal units, while an operating system or older utility may display the same bytes in binary units. A drive sold as 1 TB therefore appears as about 0.91 TiB, or roughly 931 GiB, before formatting and file-system overhead. That difference is a unit conversion, not recoverable storage.
A much larger shortfall, a drive that suddenly loses capacity, or space that disappears after connecting it to another computer can indicate an unseen partition, an unsupported file system, unallocated space, corruption, hardware errors, or a falsely labeled product. Those cases require a different investigation. Recovering apparent capacity is also separate from recovering files, and reformatting can put existing data at risk.
Why the number on the box and the number on your computer differ
Manufacturers use decimal capacity
Hard-drive manufacturers market capacity in base 10. In that system, 1 GB is 1,000,000,000 bytes and 1 TB is 1,000,000,000,000 bytes. Seagate describes this as the industry marketing convention. Apple Support documented the same decimal definitions on February 26, 2025.
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Some software uses binary units
Many operating-system displays and older disk tools calculate in powers of 1,024. A binary gigabyte (GiB) is 1,073,741,824 bytes, and a binary tebibyte (TiB) is 1,099,511,627,776 bytes. The labels may still say “GB” or “TB,” even when the calculation is binary, which makes the result look smaller than the product label.
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| Advertised decimal capacity | Approximate binary display | What the difference means |
|---|---|---|
| 1 TB = 1,000,000,000,000 bytes | About 0.91 TiB (roughly 931 GiB) | The same physical bytes expressed with a different unit |
| 1 GB = 1,000,000,000 bytes | About 0.93 GiB | A conversion, not a failed or counterfeit drive by itself |
The exact number you see varies with the operating system, the disk utility, and how the interface rounds values. Do not treat the conversion as a measured failure rate or expect every 1 TB device to display an identical figure.
Why usable space is lower after formatting
A new drive does not expose every byte as file storage. Formatting creates a file system, allocation structures, metadata, and other system areas. Apple summarizes the practical result as: “1GB = 1 billion bytes, and 1TB = 1 trillion bytes; actual formatted capacity less.”
File systems also reserve space in allocation units. A file occupies one or more of those units, so the amount reported as free can differ from a simple subtraction of file sizes. Allocation-size effects are one possible explanation listed by SanDisk Support. They generally account for a modest reduction, not an unlimited one.
For that reason, compare your computer’s figure with the expected decimal-to-binary conversion first, then allow for formatting. If the remaining gap is still unusually large, investigate the disk layout and device health instead of assuming formatting explains everything.
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When a large or changing discrepancy points to another problem
Support guidance from SanDisk identifies several possibilities. These are diagnostic categories, not a diagnosis of your particular drive.
An unallocated area is not part of a usable volume
A disk can contain bytes that are not assigned to a mounted volume. The operating system may show the physical disk’s total size while the file browser shows only the formatted volume. A partition can also exist without a drive letter or mount point, making its contents invisible in normal browsing.
The host cannot interpret a partition or file system
A partition created with a file system unsupported by the current host may appear as unknown, unreadable, or absent. The same can happen when a disk is moved between systems that use different partition or file-system conventions. In that situation, the data may still occupy space even though the current computer cannot present it as files.
Corruption or drive errors alter what is reported
File-system corruption, damaged partition information, failing media, or an unreliable connection can produce an incorrect capacity, an intermittent volume, or a disk that changes size between connections. Repeated disconnects, read errors, or a drive that becomes read-only are warning signs to preserve data before attempting repairs.
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The product may not provide its labeled capacity
SanDisk also lists fraudulent or falsely labeled products as a possible cause. A counterfeit device can report a large size while containing less physical storage, allowing old files to be overwritten when the apparent capacity is exceeded. A normal decimal/binary difference is not evidence of fraud; suspicion rises when independent capacity testing, error behavior, and the device’s physical labeling all conflict.
A safe way to classify what you are seeing
Use this order because it separates harmless display math from conditions that can endanger files.
- Record the facts. Write down the capacity printed on the enclosure, the capacity shown for the physical disk, the capacity shown for each volume, and the free-space figure. Note which computer and operating system produced each number.
- Check the unit convention. Compare a decimal label with the approximate binary equivalent. For a 1 TB label, a result near 0.91 TiB or 931 GiB is expected before normal formatting overhead.
- Look at the disk layout without changing it. In your operating system’s disk-management or disk-utility tool, observe whether the disk contains multiple volumes, unallocated space, an unmounted volume, or a file system marked unknown. Do not delete, initialize, resize, or format anything merely to make the display match the label.
- Check whether the difference follows the computer. If a volume is visible on one host but not another, file-system or partition support is a stronger possibility than a missing physical capacity. This observation does not prove the data is intact.
- Watch for health symptoms. Error messages, slow or repeating reads, unexpected disconnections, and changing capacity justify stopping experiments and prioritizing a data-preserving assessment.
- Consider authenticity only after ordinary explanations. If the unit conversion, formatting, layout, and host support do not explain the result, treat the device as potentially defective or falsely labeled and seek an appropriate verification method before trusting it with more data.
The available support material does not establish one platform-independent sequence for repairing partitions, checking a drive’s true capacity, or correcting corruption safely. Menu names and the risks of each operation differ by operating system and file system.
Can you recover the “missing” space?
If the difference is only decimal versus binary
No physical capacity is missing, so there is nothing to recover. The drive already contains the advertised number of bytes; the computer is expressing them in a different unit. You can change the display or use a calculator that labels decimal and binary units, but you cannot turn 0.91 TiB into 1 TiB without adding storage.
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If space is unallocated or a volume is hidden
It may be possible to make that space usable by creating, mounting, or resizing a volume, but doing so can alter partition structures. Whether files can be preserved depends on what is already on the disk. The cited guidance identifies unallocated space and host visibility as causes but does not establish a data-safe, universal repair procedure. Obtain instructions for your exact operating system and disk layout before committing a change.
If the file system or partition information is damaged
Capacity recovery and file recovery are separate goals. Rebuilding a partition, repairing a file system, or formatting may restore a normal-looking volume while making existing files harder or impossible to retrieve. The sources available here do not determine recovery odds for a specific damaged drive.
If the device is failing or falsely labeled
Software cannot restore physical sectors that do not exist or are no longer reliable. A replacement external drive may solve a storage-capacity problem, but it will not recover files from the original device. Preserve the original and seek qualified recovery guidance if the data matters.
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Why formatting is not a recovery method
SanDisk describes formatting and reclaiming capacity as a resolution for some full-capacity or disk-full situations. That is a capacity-management remedy, not proof that formatting preserves existing files. Formatting normally creates a new file-system structure and can remove the references needed to locate old data.
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If wanted files remain on the drive, do not format, initialize, repartition, run a “repair” that writes to the disk, or repeatedly reconnect a drive that is failing until you have authoritative, data-preserving advice. Stop using the device for new writes. The collected guidance does not provide a recovery-safe sequence, a success rate, or a verified recovery provider.
Common scenarios and the right interpretation
| What you observe | Most useful interpretation | What not to assume |
|---|---|---|
| A 1 TB drive appears near 931 GB | Likely decimal labeling viewed through a binary-style display, with formatting overhead | That 69 GB is recoverable lost storage |
| The physical disk is large, but the visible volume is much smaller | Another partition may be unmounted, unsupported, or unallocated | Formatting the visible volume will safely reveal the rest |
| The capacity changes or errors appear during reads | Corruption, hardware trouble, cable or enclosure problems are possible | A partition resize will fix the underlying device |
| The drive reports a large size but files become corrupt after filling it | A falsely labeled or defective product is possible | Replacing the drive will recover data already written to it |
What evidence is still needed for a recovery decision
A reliable answer about recovering files requires details that a capacity label cannot supply: the operating system and file system, the disk’s partition map, whether the drive mounts, whether files open, the pattern of read errors, and whether new data has been written. A capacity display alone cannot establish that files are present, deleted, overwritten, or recoverable.
For valuable data, preserve the original device in its current state and obtain advice specific to that device and symptom set. Only after files are secured should you decide whether to repartition, reformat, replace the enclosure, or replace the external drive.
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