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When a NAS reports low space, do not start by deleting the biggest folders or buying disks. First determine whether the pressure is on a volume, storage pool, shared folder, or application; then identify retained data such as snapshots, recycle bins, and backup versions. Make a separate, restorable backup before permanent cleanup or any disk, RAID, or migration work. If the data is genuinely needed, expand the existing pool only when your model and RAID layout support it, or move to a larger pool or NAS.
What “running out of storage” actually means
A NAS can show several different capacity warnings. A volume is the filesystem space presented to shared folders and applications. A storage pool is the collection of disks and RAID metadata from which a volume is created. A shared folder can be large while the volume also contains snapshots, databases, package data, and backup repositories that are not obvious in a normal folder view.
Start by recording the exact warning and its location. Note the NAS model, operating-system version, filesystem (for example, Btrfs or ext4), RAID type, number and sizes of installed drives, free bays, and whether the warning concerns one volume or the entire pool. These details determine which cleanup and expansion actions are safe. The detailed menu paths below are for Synology DSM; another vendor may use different names and may not offer the same features.
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Synology advises keeping volume usage below 80% to reduce performance problems and possible errors with packages or services. That is Synology guidance published October 1, 2026, not a universal limit for every NAS brand, filesystem, or workload. A database-heavy or virtual-machine workload may need more headroom than a mostly archival share.
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Check the dashboard or storage manager for:
- Volume utilization and free space.
- Storage-pool health, degraded arrays, and active repairs.
- Drive health alerts and incompatible or failing disks.
- Which shared folder, package, or service generated the warning.
If a pool is degraded or a disk is failing, resolve that displayed condition before attempting expansion. Synology’s add-drive procedure requires a healthy pool and an eligible unused drive; starting a rebuild while the array is already unhealthy can increase risk.
How to locate the space before deleting anything
Use DSM Usage Details when the volume qualifies
On eligible Synology DSM 7 Btrfs volumes, open Storage Manager, select the volume, and open Usage Details. The analysis can attribute space to shared folders, snapshots, the Synology Drive database, and other services. It is not available for every filesystem or volume state. Enabling the analysis can temporarily reduce performance by approximately 3%–10% while it runs, according to Synology’s DSM 7 help documentation: View the Volume Usage.
Treat the result as an inventory, not an automatic deletion list. A large database, media folder, or virtual-machine image may be essential even if it is the largest item.
Generate a Storage Analyzer report
Synology Storage Analyzer can report volume usage, shared-folder size, file groups, duplicate files, large files, recently modified files, and least-accessed files. Its DSM 7.4 capabilities are listed in the Storage Analyzer technical specifications. Create a report, sort by size, and have the owner of each dataset classify candidates as required, archival, duplicated, or disposable.
Large and old are not the same as unnecessary. A rarely accessed legal record may need to remain, while a recently created temporary export may be disposable. Open files, permissions, encryption, deduplication, and hard links can also make a file-level total differ from the apparent space used by a service.
Inspect services that retain hidden data
Synology identifies several common consumers that may not appear as ordinary user files:
- Synology Drive or Cloud Station databases and sync history.
- Snapshot Replication snapshots.
- Download Station temporary and completed downloads.
- Active Backup for Business repositories and retention versions.
- Shared-folder recycle bins.
- Hyper Backup versioned archives.
Open each relevant application and review its retention, versioning, destination, and task history. Do not assume that reducing a visible shared-folder total will recover the same amount from the volume; snapshots and service databases can continue to hold blocks after files disappear.
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What can safely reclaim space
Empty shared-folder recycle bins deliberately
Deleted files remain in a shared-folder recycle bin until they are permanently removed. Check the recycle-bin setting for each large shared folder, review its contents with the folder owner, and permanently delete only what policy allows. Synology notes that emptying a recycle bin may not reclaim space immediately, because background cleanup, snapshots, clones, or filesystem accounting can delay the visible change. Allow maintenance to finish and recheck the volume rather than repeatedly deleting more data.
Reduce snapshot retention only after checking recovery needs
A snapshot preserves references to data blocks as they existed at an earlier point. When a file changes or is deleted, blocks still needed by a retained snapshot remain allocated. The amount held depends on how much data changes between snapshots, so there is no fixed amount of space released by deleting one snapshot.
Review snapshot schedules and retention in Snapshot Replication. Deleting old snapshots or shortening retention trades historical recovery points for free space. Keep enough points to meet your recovery objective, and document the change. Synology’s guidance on reclaiming snapshot space is available at How can I free up the space consumed by snapshots?; the related explanation of snapshot behavior is in the Snapshot Replication Quick Start Guide.
Review backup versions and rotation
Versioned backups can consume more space than the current source data because each retained version preserves changes. In Hyper Backup, inspect the task’s version count, rotation mode, and Smart Recycle or custom policy. Remove or rotate versions through the backup application, not by deleting files inside its repository. The DSM 7 options are described in Data Backup.
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Check download queues, completed downloads, transcoding caches, package logs, container images, and old installers. Use the application’s own cleanup controls where available. Stopping a service before removing its data may be required; deleting an unfamiliar package directory can corrupt its database or configuration. If a service has no documented retention control, export or back up its configuration and consult the vendor’s instructions before manual removal.
Move or archive data instead of deleting it
For records that must be retained but need not stay on primary storage, move them to a separate storage medium or archive tier after verifying access and backup requirements. Preserve timestamps, permissions, encryption keys, and checksums when those properties matter. A move is not a backup until the destination is independently verified and the original is not the only remaining copy.
Back up before permanent cleanup or storage changes
Before emptying bins, removing snapshots, changing retention, replacing disks, changing RAID, or migrating volumes, confirm that important data exists in a separate, usable backup. Synology describes the 3-2-1 strategy as three copies of data, on two storage media, with one copy off site. Its backup guidance is at How can I back up my Synology NAS?.
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Possible Synology tools include Hyper Backup, Snapshot Replication, Active Backup for Business, USB Copy, and Cloud Sync. Choose according to the data type and destination. A USB disk can be a practical backup target, but it is not protection until the job completes and a sample restore succeeds. RAID redundancy, snapshots, and a second local copy reduce some failure scenarios; none is a complete independent backup by itself.
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Choose between cleanup and expansion
| Route | Problem it addresses | Compatibility to verify | Main trade-off |
|---|---|---|---|
| Delete or move unnecessary data | Unneeded files, temporary data, or obsolete exports | Ownership, retention policy, permissions, and restore requirements | Fastest and usually cheapest, but deletion is irreversible without a backup |
| Reduce recycle-bin, snapshot, or backup retention | Historical copies retaining blocks | Recovery objectives and application-specific retention controls | Recovers space at the cost of older recovery points |
| Add compatible drives to the existing pool | Insufficient physical capacity with free bays | NAS model, RAID type, filesystem, drive type, compatibility list, and pool health | Requires rebuild time and leaves the existing layout in place |
| Replace drives with larger ones | Full bays but a supported upgrade path | Exact RAID rules, replacement order, minimum number of larger drives, and backup | Multiple rebuild cycles, cost, and exposure during each replacement |
| Add an expansion unit or another pool | Existing pool cannot expand in place | Model-specific expansion support, ports, filesystem, and application placement | Additional hardware and more complex management |
| Migrate to a larger-bay NAS or rebuild a larger pool | Long-term capacity or performance redesign | Migration path, downtime, backup capacity, and destination compatibility | Highest planning and restore effort, but often the cleanest long-term result |
Compare usable capacity, not the sum of drive labels. RAID parity, mirroring, filesystem overhead, reserved space, and mixed-drive rules reduce the capacity available to files. Use the capacity calculator appropriate to your NAS platform; Synology recommends its RAID calculator when planning an expansion.
Expanding a Synology pool by adding drives
Synology’s current expansion overview lists drive-addition expansion for JBOD, RAID 5, RAID 6, RAID TP, RAID F1, SHR-1, and SHR-2, subject to model and configuration limits. Start with the current How do I expand the storage pool, dual pool, or volume capacity? guidance and the DSM 7 procedure Add Drives to Expand the Storage Pool Capacity.
- Confirm the pool is healthy. Resolve degraded status, drive warnings, or an active repair before adding hardware.
- Check the model’s compatibility list. Confirm the drive type, interface, capacity, firmware, and supported bays. A physically fitting disk is not automatically an eligible disk.
- Verify an unused bay and a compatible RAID layout. The available expansion choices depend on the existing pool, not just the new drive’s size.
- Back up and test the backup. A rebuild stresses every existing drive and can expose a second failure.
- Insert the eligible drive and open Storage Manager. Select the storage pool and use the expansion action shown by DSM; do not initialize the disk as an unrelated pool if the goal is to enlarge the current one.
- Confirm the new usable capacity and start the expansion. Read the warning about downtime, performance, and data protection before accepting.
- Wait for repair, parity calculation, or resynchronization to finish. Avoid unplugging drives, forced shutdowns, or heavy workloads during the operation. Recheck pool and volume health afterward.
DSM may reject a drive that is too small, unsupported, or otherwise ineligible. Do not bypass that warning by destroying the pool. If the pool cannot expand, use one of the alternatives below.
Replacing drives with larger ones
Drive replacement can increase capacity when all bays are occupied, but the exact rules vary by RAID type and drive arrangement. Synology’s overview explains that some RAID types require replacing every same-size drive before the additional capacity becomes available, while SHR-1 and SHR-2 have their own minimum replacement counts and conditions. There is no safe, universal “replace one disk and immediately gain space” sequence.
- Identify the exact RAID type, current drive sizes, and the minimum larger-drive count required by the current Synology instructions.
- Confirm the replacement model is supported and at least as large as the disk it replaces; account for manufacturers’ slightly different reported capacities.
- Make and verify a separate backup, then schedule the work for a period when reduced performance is acceptable.
- Replace only the drive specified by DSM, start the repair, and wait until the pool reports a healthy, completed rebuild.
- Repeat for the next drive only after DSM’s instructions say the array is ready. Do not remove a second drive during an active repair.
- After the required number of drives has been replaced, expand the pool or volume using Storage Manager and verify the new usable capacity.
If the documentation for your exact model does not show a supported replacement path, stop. A larger disk installed without a supported expansion method may remain unused or require a new pool and restore.
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When the existing pool cannot expand
Use a compatible expansion unit
An expansion unit can extend capacity only when the NAS model and the unit are explicitly compatible. Check the current model-specific list before purchasing, and confirm whether the resulting space is available to the existing pool, a separate pool, or only selected volumes and packages.
Create another pool or volume
A second pool can hold archives, backups, or less latency-sensitive data without changing the first pool’s RAID layout. Plan application paths, shared-folder permissions, quotas, snapshots, and backup jobs so that new data does not silently remain unprotected.
Migrate to a larger-bay model
A migration can provide more bays and a cleaner long-term layout, but compatibility, downtime, licensing, package support, and network identity must be checked. Keep the original NAS intact until users can open files, scheduled tasks run, and backups complete on the destination.
Back up, build a larger pool, and restore
When an in-place expansion is unsupported, this is the most flexible route. It requires enough temporary backup capacity for all required data, a new pool configured with the desired RAID and filesystem, and time to restore shares, permissions, applications, snapshots, and backup jobs. Treat the old pool as the source until the restored system has been verified.
Common symptoms and the right response
- The dashboard says full, but shared folders do not. Check snapshots, recycle bins, service databases, backup repositories, and package data; visible shares are not the whole volume.
- Deleting files changed nothing. Empty recycle bins, review snapshots and clones, allow background cleanup, and recheck after maintenance. Do not assume the deletion failed or immediately delete more.
- Storage Analyzer itself makes the NAS slow. Its usage analysis can temporarily reduce performance by about 3%–10% on eligible volumes. Schedule it during a quiet period and wait for completion.
- A new disk is rejected. Recheck model compatibility, disk type, capacity, bay assignment, pool health, and the RAID’s expansion rules. Do not force the disk into the pool.
- The pool is degraded. Follow the displayed repair or replacement instructions before capacity work. Expansion is not a substitute for restoring redundancy.
- A backup job is the largest consumer. Inspect version rotation and retention in the backup application. Deleting repository files manually can make every remaining version unusable.
- There is no safe cleanup candidate. Stop deleting and plan capacity expansion or migration. Preserve recovery points and obtain the required backup space first.
How to prevent the next storage emergency
- Set alerts below the point at which services become unreliable; for Synology, use the vendor’s below-80% guidance as a planning signal rather than waiting for 100%.
- Review snapshots, recycle bins, sync history, and backup versions on a schedule, with named owners for each retention policy.
- Track growth by shared folder and application so that a rapidly expanding dataset is visible before it fills the volume.
- Keep a tested 3-2-1 backup and document where encryption keys, application exports, and restore instructions are stored.
- Before buying drives, check the exact NAS compatibility list and calculate usable RAID capacity, rebuild time, and backup requirements.
- Leave operational headroom for updates, logs, databases, snapshots, and temporary files; free space is part of service reliability, not wasted capacity.
What readers using another NAS brand should do
The diagnostic logic is portable, but menu names, filesystem behavior, RAID support, and expansion rules are not. Use your manufacturer’s storage manager to distinguish volume, pool, and service usage; identify recycle bins, snapshots, sync databases, and backup retention; and read the model-specific disk-compatibility and expansion documentation. Do not apply Synology’s RAID or replacement sequence to another platform without an explicit equivalent instruction.
The safest order remains the same: establish health, identify the real consumer, protect the data with a tested separate backup, reclaim only policy-approved data, and expand or migrate according to the exact pool configuration.
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