If a NAS cannot see a drive, first check model compatibility and installation, then isolate the drive from the bay. If the capacity looks too small, identify whether you are viewing the physical disk, a RAID group or storage pool, a volume, or client free space: each reports a different amount. Do not initialize, format, remove pool members, or start a rebuild until you understand the effect on your data.
If the NAS does not detect the drive
Work from the simplest explanations toward hardware isolation. Check the NAS model’s instructions before moving a disk: hot-swap support and safe slot-movement procedures vary by model. Synology’s troubleshooting guidance applies to DSM 7 series, legacy DSM versions, and DSM Enterprise; use your own vendor’s manual for other systems. Synology’s undetected-drive steps are one model-family-specific reference.
- Check the installation. Confirm the drive and tray are seated as the manual specifies, and inspect the tray and connectors for visible damage. Synology notes that rack-model tray screws should be tightened. Do not remove or insert a drive while powered on unless the NAS explicitly supports hot swapping.
- Check compatibility for the exact NAS model. Look up the drive in the vendor’s model-specific compatibility list. A drive may be detected but marked incompatible, unverified, or unrecognized; the vendor may also recommend updating its drive database. Synology explains these status distinctions in its incompatible or unverified drive guidance.
- Isolate the bay, if it is safe to do so. If an empty bay is available, move the drive there following the manual. If it appears in the other bay, the original slot or bay is a likely source of the problem. If no bay is free, use the manufacturer’s slot-isolation procedure; do not improvise by removing an active pool member.
- Test the drive on a computer, if practical. If it is not recognized there either, the drive itself may be faulty. If it works in another NAS bay, contact the NAS vendor or reseller about the original slot. These are diagnostic clues, not a guarantee: another computer’s connectors, power, or operating-system support can affect the result.
- If detected but unavailable for an operation, check pool rules. A drive can appear in the interface yet be ineligible for a particular replacement or expansion. Check its health, supported type and capacity, whether it is already in use, and any slot or pool-function restrictions. Synology’s pool expansion and replacement requirements depend on the configuration.
Find which capacity figure is smaller than expected
Compare the same layer with the same layer. A drive’s reported size, a RAID group or storage pool’s usable capacity, an allocated volume’s size, and a client’s free space are not interchangeable. Check the NAS storage manager and note exactly where the unexpected figure appears before changing any settings.
- Drive list: usually reflects the physical disk’s reported capacity, subject to how the interface displays units.
- RAID group or storage pool: reflects the combined drives after redundancy and system-level reservations.
- Volume: reflects the capacity allocated to that filesystem, which can be less than the pool if some pool space is unallocated.
- Client, such as Windows Explorer: free space generally reflects the shared data volume available to that client, not the entire storage pool.
Account for units, RAID, and reserved space
Decimal drive labels versus binary display units
Drive makers generally label capacity in decimal units, while some interfaces display binary-scaled amounts. The bytes have not necessarily disappeared; the numerical value changes with the unit convention. QNAP’s FAQ gives a 2 TB decimal-labelled disk—2,000,000,000,000 bytes—as an illustrative example that displays as 1.81 when divided using 1024-based units. This is a unit-conversion example, not a promise about how every NAS labels its figures. QNAP’s capacity FAQ and Seagate’s explanation of advertised and displayed drive size describe the distinction.
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RAID protection and drive sizes
Redundancy reduces usable capacity. In QNAP’s general guidance, RAID 1 provides usable capacity equivalent to one member drive, RAID 5 to N−1 drives, and RAID 6 to N−2 drives; the arithmetic uses the smallest member drive. Larger drives in a mixed-size set can therefore leave some capacity unused. These are general RAID calculations, not a substitute for your NAS model’s pool calculator or configuration rules. QNAP’s RAID capacity FAQ explains the examples.
System, RAID, and filesystem reservations
NAS software may reserve space for system configuration, pool and RAID metadata, and filesystem metadata. The amount is vendor- and configuration-specific, so these figures are useful context rather than a guaranteed deduction for every NAS:
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- QNAP QTS and QuTS hero: its FAQ, last modified August 27, 2026, describes typical reservations of about 70 GB per storage pool and 20 GB per static volume; about 1% of each RAID group’s capacity, up to 150 GB per group; and roughly 0.8%–1.8% of volumes over 100 GB for EXT4 filesystem metadata. QNAP says, “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.” See QNAP’s reserved-space FAQ.
- Synology calculator estimates: its calculator guidance estimates approximately 10 GB of system space reserved per drive, plus metadata reservations of 4% for Btrfs volumes and 2% for ext4 volumes. Actual capacity depends on the model and configuration, and Synology directs users to Storage Manager for the actual available volume capacity. See the Synology RAID calculator.
When pool capacity exceeds volume or client capacity
A pool can contain space that is not assigned to a volume. If the client reports less capacity than expected, inspect the data volume’s size in the NAS interface and compare it with the pool’s available space. Resizing a volume may be possible, but the supported procedure depends on the NAS, filesystem, and pool configuration. QNAP explains why a mapped client’s free-space reading corresponds to the data volume rather than the whole pool in its client free-space FAQ.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Before replacing a drive or expanding a pool
Do not choose a replacement based on capacity alone. Check the exact NAS model’s compatibility list, drive type and interface, pool capacity requirements, health status, and whether the goal is replacing a member, expanding a pool, or creating a separate volume. A physically detected drive may still be unsupported for the intended operation. Before replacing a member, back up important data and follow the procedure that matches the pool’s protection status; Synology explicitly warns that replacement steps depend on that status in its drive compatibility guidance.
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The right diagnosis depends on the NAS model, drive model, pool or RAID layout, and the interface showing the unexpected capacity. If those details do not explain the symptom, provide them to the NAS vendor or reseller before making destructive storage changes.
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