Hard Drives
Hard Drive Capacity Explained: Why 24 TB Can Appear as 21.8 TiB
A smaller number is not always missing storage. Convert TB to TiB, then separate RAID protection, volume capacity and the space used by files and services.
A 24 TB drive contains about 21.83 TiB because TB and TiB count the same bytes using different units. That conversion alone is not lost data or a faulty disk. RAID protection, filesystem overhead, system reservations and stored data are separate deductions. Check which layer the screen is showing before comparing it with the number on the box.
You install a new drive, open the storage screen and see a smaller number. Before returning it or changing the partition, ask what the number represents: the physical drive, a protected storage pool, a created volume or the space currently free.
Those are not interchangeable. Reading them separately makes both a new purchase and an unexpectedly full NAS easier to understand.
TB and TiB Describe the Same Bytes Differently
Drive manufacturers normally use decimal units: 1 TB = 1,000,000,000,000 bytes. A tebibyte is binary: 1 TiB = 1,099,511,627,776 bytes, or 240 bytes.
Divide the byte count by the chosen unit. For a nominal 24 TB drive, 24,000,000,000,000 ÷ 1,099,511,627,776 = 21.8279 TiB. Rounded to one decimal place, that is 21.8 TiB. Nothing in this calculation has removed a file or reserved space.
| Labelled decimal capacity | Same bytes expressed in TiB, approximately |
|---|---|
| 4 TB | 3.64 TiB |
| 8 TB | 7.28 TiB |
| 12 TB | 10.91 TiB |
| 16 TB | 14.55 TiB |
| 24 TB | 21.83 TiB |
Software labels are not perfectly consistent. Some interfaces display binary quantities while using a “TB” label; others use decimal units. Read the documentation and byte count where available instead of assuming every operating system or NAS screen uses the same convention.
Read the Drive, Array and Volume Separately
Read Capacity at the Right Layer
Do the unit conversion before subtracting space for protection and storage services.
01 One Drive
24 TB ≈ 21.83 TiB24,000,000,000,000 bytes divided by 1,099,511,627,776 bytes per TiB.
02 Protected Array
4 × 24 TB RAID 5 ≈ 65.48 TiBSimplified equal-drive arithmetic: three drives of data capacity, before overhead.
03 Created Volume
Check Total and Free SpaceRead the pool, volume, snapshots and actual available capacity separately.
Start with the physical-drive inventory: model, nominal capacity and whether every intended drive is detected. Then read the storage-pool or array configuration. A disk allocated to a hot spare, a mirror or parity does not contribute to data capacity in the same way as an unprotected disk.
Next inspect the volume. Its allocation, filesystem and system reservations affect the size the user can access. Finally compare total volume capacity with free or available space. Existing files, snapshots and other retained data belong at that final stage.
This order avoids subtracting the same thing twice. For example, a storage screen may already show a protected pool in binary units. Applying another TB-to-TiB conversion to that displayed number would make the estimate wrong.
Worked Example: Four 24 TB Drives
Four nominal 24 TB drives contain 96 TB of raw decimal capacity. The following examples use equal-size drives and simplified conventional RAID arithmetic. They exclude filesystem overhead, system reservations, spare disks and implementation-specific limits.
| Configuration | Ideal decimal data capacity | Same capacity in TiB, approximately |
|---|---|---|
| Raw total before protection | 96 TB | 87.31 TiB |
| RAID 5: one-drive-equivalent parity | 72 TB | 65.48 TiB |
| RAID 6: two-drive-equivalent parity | 48 TB | 43.66 TiB |
| RAID 10: two mirrored pairs | 48 TB | 43.66 TiB |
Parity is distributed across the RAID array; “one-drive-equivalent” describes the capacity cost, not a dedicated parity drive in RAID 5. Real usable capacity will be lower once the storage system’s additional requirements are included.
Use the RAID calculator for planning comparisons, then confirm the selected NAS, drive compatibility and intended configuration. The RAID comparison guide explains protection trade-offs. RAID improves availability under specified failures; it is not an independent backup.
When the Difference Is Too Large for Unit Conversion
A 24 TB drive showing only a small fraction of its capacity needs a different investigation. First distinguish a physical device reporting a small size from a full-size device containing a smaller partition.
- Device identity: confirm the model and the capacity the system detects, rather than relying only on a retail description.
- Connection: verify that the enclosure, USB bridge, controller and software support the drive’s capacity.
- Partition and volume: check for unallocated space or an allocation smaller than the underlying device.
- NAS configuration: check missing members, spare allocation, pool limits and any expansion that has not completed.
Protect existing data before altering partitions or storage pools. Formatting or reinitialising is not a way to reverse unit conversion, and can erase the information you were trying to recover.
If the hardware inventory itself reports an unexpected identity or size, retain the purchase and device details for the Support Specialist before continuing.
Total Capacity and Free Space Answer Different Questions
A correctly sized volume can still run out of space. That does not mean its TB-to-TiB conversion changed overnight.
Check what is consuming the volume: live files, backup versions, snapshots, recycle bins, application data and any reserved area. Deleting a visible file does not always remove every retained version immediately. Conversely, increasing a snapshot allowance is not the same as adding another physical drive.
The NAS space-after-deletion guide explains those checks. Start there when total capacity is correct but free space is unexpectedly low. Do not remove recovery points blindly just to make a graph look better.
Allow operating headroom for the workload. A plan that fits today’s files exactly may leave no room for growth, temporary work or the retention policy that makes recovery useful. Define how much data must be retained and for how long before choosing the next capacity step.
Choose Capacity Around the Complete Storage Plan
Family photograph shown. Select the required capacity and verify its exact model against the intended NAS before ordering.
Write the requirement in usable terms: current data, expected growth, protection level, recovery retention and operating headroom. Then translate that into drive count and capacity using the system’s supported configuration.
For example, “four 24 TB drives” is a component list. “A protected pool for the working dataset, with separate recovery copies and room for the required versions” is a storage requirement. The latter is much easier to evaluate against a real workflow.
The practical answer: 24 TB becoming about 21.83 TiB is normal unit conversion. A protected array becoming smaller again can also be expected. An unexpectedly small detected device, incomplete allocation or disappearing free space needs its own check at the correct layer.
Frequently Asked Questions
Is a 24 TB drive really only 21.8 TB?
A nominal 24 TB byte count is about 21.83 TiB. TB and TiB use different units. Some software uses a TB label for a binary quantity, so check the interface’s convention.
Does formatting cause the whole TB-to-TiB difference?
No. Unit conversion describes the same bytes differently before any filesystem deduction. Formatting, metadata and system reservations can reduce usable space further.
Why do four 24 TB drives not provide 96 TB for files in RAID 5?
Simplified equal-drive RAID 5 uses one drive-equivalent for parity, leaving 72 TB, about 65.48 TiB, before overhead. The raw 96 TB total includes the capacity used for protection.
Can I format the drive to get the missing conversion space back?
No. Changing the filesystem does not change the decimal-to-binary relationship. Reformatting may destroy existing data and should not be used to fix a unit-label misunderstanding.
What if the drive reports much less than the expected binary value?
Check the detected model and device capacity, enclosure or controller support, partition allocation and NAS pool configuration. Protect data before making structural changes.
Why is free space lower even when total capacity looks correct?
Files, retained versions, snapshots, recycle bins, applications and reservations can consume the volume. Investigate that usage separately from unit conversion and RAID protection.
