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Scan for outdated or missing drivers - takes under a minuteDriver Scan →Repair Windows errors before they cause bigger problemsFix Now →If a NAS drive is missing or its capacity looks too small, first identify whether the issue is the drive, its bay or compatibility—or simply a difference between drive, pool, volume and client figures. Check detection without changing the storage configuration, then compare capacity at the same layer. Do not initialize, format, remove pool members or start a rebuild until you understand the effect on data protection.
If the NAS does not detect the drive
Work from the simplest checks toward isolating the hardware. Menu names and safe drive-movement procedures vary by NAS model, so follow that model’s manual. Synology’s steps apply to its DSM 7 series, legacy DSM versions and DSM Enterprise, and should not be assumed to describe every vendor’s interface.
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- Check installation. Confirm the drive is seated as the NAS manual specifies. Look for damage to the tray or connectors. Synology advises tightening tray screws on rack models. Do not remove or insert a drive while powered on unless the model supports hot-swapping.
- Check exact-model compatibility. Look up the drive against the compatibility list for your NAS model. Vendors may label a drive incompatible, unverified or unrecognized if requirements are not met. If the vendor recommends updating its drive database, do so using its documented procedure. See Synology’s compatibility list and its undetected-drive troubleshooting guide.
- Try another bay, if safe. If an empty bay is available, use the manual’s procedure to test the drive there. If there is no empty bay, follow the vendor’s prescribed slot-isolation steps; power down first if the manual requires it. A drive detected in another bay points toward a problem with the original slot or bay.
- Test the drive on a computer, if practical. If the computer also cannot find it, the drive may be faulty. If it works in another NAS bay, contact the NAS vendor or reseller about the original bay. These outcomes are diagnostic clues, not universal rules for every NAS status label.
- Check why a detected drive cannot be selected. Detection does not mean the drive qualifies for every pool operation. Check its health, drive type and capacity; whether it is already in use; and any slot or function restrictions. Synology’s pool expansion requirements are configuration-specific.
Before replacing a drive, check its health and compatibility along with the pool’s requirements. Synology warns users to back up data and follow the procedure appropriate to the pool’s protection status when replacing an incompatible or unverified drive. See Synology’s guidance on incompatible or unverified drives.
Find which capacity figure looks wrong
A NAS reports capacity at several layers, and the numbers are not interchangeable. Identify exactly where the unexpected figure appears before changing settings.
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- Drive list: capacity reported for an individual physical disk.
- RAID group or storage pool: capacity available after combining drives and applying protection.
- Volume: capacity allocated from a pool to a filesystem or share.
- Client free space: space available to a connected computer, which may reflect the data volume rather than the whole pool.
For the pool and volume figures, check the NAS storage manager. Synology’s RAID calculator provides estimates and directs users to Storage Manager for actual available volume capacity: Synology RAID calculator. A mapped client’s free-space reading may correspond to the data volume, not all space in the pool; see QNAP’s explanation of network-drive capacity in Windows Explorer.
Why a drive or volume can show less capacity
Decimal and binary units
Drive labels generally use decimal units, while some NAS and computer interfaces represent capacity using binary-scaled units. The same byte count can therefore appear smaller when expressed using the other convention. QNAP’s FAQ gives the example of a disk sold as 2 TB, or 2,000,000,000,000 bytes, displaying as 1.81 when divided using 1024-based units. That is an illustrative conversion from QNAP’s FAQ, last modified March 20, 2017—not a guarantee that every interface labels units the same way. Seagate also explains the difference between advertised and displayed capacity in its drive-capacity article.
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RAID protection and drive-size limits
RAID redundancy uses some raw drive capacity for protection. In QNAP’s general capacity guidance, RAID 1 provides usable capacity equivalent to one drive, RAID 5 to N−1 drives, and RAID 6 to N−2 drives, using the smallest member drive for the calculation. These are general arithmetic rules, not a model-specific estimate; configuration and implementation matter. If drives have different capacities, some space may remain unused depending on the RAID configuration. See QNAP’s RAID capacity FAQ.
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System, RAID and filesystem reservations
Even after accounting for unit conversion and RAID, the pool or volume may show less than expected because the NAS reserves space for system data, RAID metadata and filesystem metadata. The reservation depends on vendor, model, configuration and filesystem.
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| Vendor estimate or guidance | Reservation described | Qualification |
|---|---|---|
| QNAP storage-reserved space | About 70 GB per storage pool and 20 GB per static volume | Typical values for QTS and QuTS hero, not universal; QNAP FAQ last modified August 27, 2026. |
| QNAP RAID-reserved space | About 1% of each RAID group’s capacity, up to 150 GB per group | Typical values for QTS and QuTS hero, not universal; QNAP FAQ last modified August 27, 2026. |
| QNAP EXT4 filesystem metadata | Roughly 0.8%–1.8% of volumes over 100 GB | QNAP’s typical guidance for QTS and QuTS hero, not a fixed amount; FAQ last modified August 27, 2026. |
| Synology calculator system-space estimate | Approximately 10 GB per drive | Estimate in Synology’s calculator guidance; actual capacity varies by model and configuration. |
| Synology filesystem metadata estimate | 4% for Btrfs volumes; 2% for ext4 volumes | Calculator guidance, not a universal measurement; actual volume capacity can differ. |
QNAP says that “QTS and QuTS hero automatically reserve part of your disk space as System Reserved space, used to store configuration information and metadata for your storage pools and volumes.” The associated figures are typical estimates, not a promise for every NAS. See QNAP’s reserved-space FAQ and Synology’s calculator notes.
Unallocated pool space
A volume may use only part of its storage pool. If the pool has free space but the volume or a mapped client shows less capacity, check the volume’s allocation in the NAS storage interface. Resizing may be possible, but use the NAS’s supported procedure for the current pool and volume configuration. Do not assume that free pool space is already assigned to a volume.
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Before you replace, add or expand a drive
A detected drive may still be unsuitable for a particular expansion or replacement. Verify the exact NAS model’s compatibility list and the pool’s rules before proceeding. Check:
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- whether its type and capacity meet the pool operation’s requirements;
- whether it is already assigned to another pool or function;
- whether the slot supports the intended operation; and
- whether you are replacing a member, expanding a pool or creating a separate volume.
Compatibility and expansion rules vary by model and configuration, so there is no safe universal replacement recommendation without the NAS model, drive model and current pool layout. Back up important data and confirm the pool’s protection status before any operation that could change its members.
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