When a NAS is nearly full, do not immediately delete large files or buy bigger drives. First identify whether the space is being used by ordinary files, recycle bins, snapshots, backup versions, or an application database; then check the health and expansion limits of the storage pool. On Synology systems, the vendor recommends keeping volume usage below 80% because higher usage can contribute to performance problems and errors with packages or services. That is Synology guidance, not a universal limit for every NAS brand or workload.
The workflow below uses Synology DSM terminology and documentation, especially for DSM 7 and eligible Btrfs volumes. Other vendors have equivalent concepts, but menu names, supported RAID layouts, filesystems, and expansion rules differ.
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Start with a safety check, not a deletion spree
A full volume can make services fail, backups stop, databases become unreliable, or updates run out of working space. Before changing anything, record the warning exactly: is it for a volume, a storage pool, one shared folder, or a particular application?
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- Open the NAS dashboard or storage manager and check the volume percentage, pool status, and individual drive health.
- Look for degraded, crashed, read-only, or repairing states. Synology’s add-drive procedure requires a healthy pool and an eligible drive; expansion is not the first response to an unhealthy pool.
- Do not remove a disk, change RAID type, or start a rebuild until you have confirmed a separate, usable backup of important data.
Synology recommends the 3-2-1 approach: three copies of data, on two storage media, with one copy off-site. RAID redundancy, snapshots, and a second local copy can all help with recovery, but none is a complete substitute for an independent backup.
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For Synology backup options, see Synology’s backup guidance. It discusses Hyper Backup, Snapshot Replication, Active Backup for Business, USB Copy, and Cloud Sync; the suitable choice depends on what you are protecting and where the destination is located.
Find what is consuming the space
Use volume and pool status first
A shared-folder total is not necessarily the whole story. A volume can also contain snapshots, package data, databases, indexes, backup versions, and hidden system data. Check the pool and volume before interpreting any file-level report.
Inspect DSM Usage Details when available
On supported Synology DSM 7 Btrfs volumes, Storage Manager’s Usage Details can break usage down by service. Depending on the system, the view can include shared folders, snapshots, the Synology Drive database, and other services. The analysis is not available for every filesystem or volume state. Enabling it can temporarily reduce performance by approximately 3%–10% while the analysis runs; that estimate applies to the analysis operation, not to normal NAS performance. The DSM View the Volume Usage documentation describes the supported conditions.
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Storage Analyzer can report volume use, shared-folder size, file groups, duplicate files, large files, recently modified files, and least-accessed files. Use these reports to create a review list. A large or old file is not automatically disposable: it may be a needed archive, a source for a backup, or a file subject to a retention requirement. The Storage Analyzer specifications describe the report categories.
Check applications that retain data
Synology identifies several services that can account for substantial use:
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- Synology Drive or Cloud Station databases and version history
- Snapshot Replication snapshots
- Download Station downloads and incomplete files
- Active Backup for Business backup data
- Shared-folder recycle bins
- Hyper Backup versioned archives
Inspect each service’s retention and destination settings rather than assuming the visible files in a shared folder represent all of its storage.
Why deleting files may not free space immediately
Shared-folder recycle bins
Deleting a file through a network share often moves it to that share’s recycle bin. Until it is permanently removed, the file still occupies space. Emptying a bin may not reclaim capacity instantly if the NAS is processing a large number of entries or if other retained data references the same blocks. Confirm the deletion completed and recheck the volume after processing finishes.
Snapshots preserve older blocks
A snapshot records references to data blocks at a point in time. When a file changes or is deleted, older blocks can remain preserved because a snapshot still needs them. The amount of space consumed depends on how much data changes between snapshots, so there is no fixed number of gigabytes per snapshot and deleting one snapshot does not guarantee a predictable amount of free space.
Review snapshot schedules and retention in Snapshot Replication. Reducing retention can recover space, but it also removes historical recovery points. Synology explains this behavior in How can I free up the space consumed by snapshots? and the Snapshot Replication Quick Start Guide.
Backup versions and application history
Versioned backups can be much larger than the current dataset. Hyper Backup can rotate old versions automatically with Smart Recycle or a customized policy. Before deleting an archive, review the retention policy, identify which restore points remain, and make sure the remaining versions satisfy your recovery needs. The DSM Data Backup documentation covers version rotation.
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A safe cleanup sequence
Work from reversible changes to irreversible ones. After each major change, allow indexing, deletion, or snapshot processing to finish and then recheck the volume.
- Record the current state. Note volume usage, pool health, drive alerts, and which services are affected. If the pool is degraded or read-only, follow the displayed recovery procedure before cleanup.
- Confirm a separate backup. Verify that a recent backup exists at another destination and that at least a sample file can be restored. A USB disk is useful only when the backup completed and can be read; plugging in a disk without verifying the job is not a backup plan.
- Review the largest consumers. Use Usage Details or Storage Analyzer where supported. Sort candidates by size, but classify each item as disposable, archivable, required, or unknown before touching it.
- Empty only approved recycle bins. Coordinate with other users first. Permanently deleting a shared-folder bin is irreversible from the NAS interface.
- Audit snapshots. Identify which shared folders and schedules retain the most history. Remove obsolete snapshots or shorten retention only after deciding how much historical recovery the business or household still needs.
- Audit backup and sync history. Check Hyper Backup versions, Synology Drive history, Cloud Station data, Active Backup repositories, and download queues. Change rotation or retention instead of deleting the newest usable recovery set.
- Remove or move confirmed disposable data. Use an external backup destination, another volume, or an archive location when the data must be retained. Do not use “least accessed” or “oldest” as the sole deletion rule.
- Recheck services and free space. Confirm that applications, indexing, scheduled backups, and shared folders work normally. If usage remains unexpectedly high, return to the service-level breakdown rather than repeating broad deletions.
Decide whether cleanup is enough
Cleanup addresses unnecessary or retained data. It does not solve a dataset that has permanently outgrown the pool. Compare the options against compatibility, usable capacity after redundancy, safety, time, disruption, and total cost, including any backup destination needed.
| Route | When it fits | What it changes | Main trade-off |
|---|---|---|---|
| Delete or move disposable data | Reports show reclaimable files, recycle-bin contents, or obsolete application data | Reduces current usage without changing the pool | Permanent deletion can remove needed data; moving data requires a reliable destination |
| Adjust snapshot or backup retention | Historical versions consume a significant share of the volume | Recovers blocks or archive space by keeping fewer recovery points | Less historical recovery coverage |
| Add compatible drives | The pool has an available bay and a supported expandable RAID type | Grows the existing pool after expansion and any required processing | Drive compatibility, redundancy rules, and rebuild time apply |
| Replace drives with larger ones | The pool supports staged replacement and the required number of drives can be upgraded | Increases capacity after each replacement and rebuild sequence completes | Requires multiple drive purchases in some layouts and prolonged rebuild exposure |
| Add an expansion unit | The exact NAS model supports a compatible expansion chassis | Adds supported drive bays outside the main enclosure | Model-specific compatibility and additional hardware cost |
| Create another pool or volume | The NAS has unused bays or suitable storage that cannot be added to the existing pool | Provides separate capacity for selected shares or services | Data placement and management become more complex; capacity is not automatically merged |
| Migrate to a larger-bay NAS | The current enclosure is full or its pool cannot be expanded economically | Moves data to a new platform with more drive capacity | Requires migration planning, backup space, and possible downtime |
| Back up, rebuild, and restore | The current layout cannot expand but a larger pool is practical | Creates a new pool with a different drive layout or filesystem | Needs enough backup capacity and time to restore everything |
Synology expansion paths and their limits
Adding drives to an existing pool
Synology’s current expansion overview lists drive-add expansion for JBOD, RAID 5, RAID 6, RAID TP, RAID F1, SHR-1, and SHR-2, subject to model, drive, and configuration rules. The DSM 7 help page requires a healthy unused drive of the same type and warns that compatibility must be checked for the specific NAS model. See How do I expand the storage pool, dual pool, or volume capacity? and Add Drives to Expand the Storage Pool Capacity.
An available bay alone is not sufficient. Confirm the drive list, interface type, minimum size, RAID level, and whether the resulting capacity will be usable after redundancy. Synology recommends using its RAID calculator to estimate post-expansion capacity instead of adding the labeled capacities of all disks.
Replacing drives with larger models
Drive replacement can expand a supported pool, but it is not a universal “swap one disk and gain space” operation. Synology states that requirements vary by RAID type and drive configuration. For some RAID types with same-size drives, every drive must be replaced; SHR-1 and SHR-2 have their own minimum replacement counts and additional conditions.
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Follow the exact instructions for the model and pool. Replace only the specified drive, wait for repair or rebuild to finish, verify the pool is healthy, and then continue if another replacement is required. Keep the backup available throughout: a rebuild places additional work on the remaining drives and does not protect against every failure.
When the existing pool cannot expand
Synology lists four alternatives: attach a compatible expansion unit, create another pool or volume, migrate to a model with more bays, or back up the data and restore it into a newly created larger pool. Expansion-unit support is model-specific, so check the NAS compatibility information before purchasing hardware. Choose among these routes based on the capacity gained, RAID and filesystem compatibility, expected downtime, migration effort, and the storage needed for a complete backup.
Keep the capacity problem from returning
- Set alerts before the volume reaches the critical range; for Synology, treat the vendor’s below-80% recommendation as an operating target rather than waiting for 100%.
- Give shared folders and applications explicit retention limits for recycle bins, snapshots, sync history, and backups.
- Review Storage Analyzer reports on a schedule, especially after adding a new backup job, camera archive, sync library, or download service.
- Separate fast-changing backup repositories and snapshot-heavy workloads when the pool design permits it, so one service does not silently consume all available space.
- Recalculate usable capacity whenever you change drive sizes or RAID layout; redundancy means the pool’s usable capacity is less than the sum printed on the drive labels.
- Test restores periodically. A backup that cannot be opened or restored cannot safely be counted as available capacity protection.
If you use a non-Synology NAS
The principles are vendor-neutral: identify the exact consumer, check pool health, verify an independent backup, clean retained data carefully, and expand only within the documented limits of the model, RAID implementation, filesystem, and drive list. Look for your vendor’s equivalents of storage-pool status, per-service usage, recycle-bin management, snapshot retention, backup-version rotation, and RAID expansion. Do not copy Synology’s 80% threshold or its drive-add rules to another platform without manufacturer documentation.
Sources and current documentation
The procedures and qualifications above are based on the following Synology documentation: storage expansion overview (updated September 17, 2026), DSM 7 add-drive help, storage-usage guidance (updated October 1, 2026), volume-usage help, snapshot-space guidance (updated March 16, 2026), and NAS backup guidance (updated May 8, 2026).
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