My Synology NAS Is Running Out of Space: What Should I Do?
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Check what is filling the volume before you buy anything: Recycle Bins, snapshots and file clones can hold space, and a volume can grow into free pool space at no cost. If the pool itself is full, add drives to empty bays or replace the drives with larger ones, one at a time. An expansion unit is an option only on models Synology lists for one.
Check your situation
Work it out for your hardware
Synology DS923+: 4 bays, drives up to 24TB each, from Synology DS923+ datasheet. Drive compatibility for this model.
Path 1: swap drives one at a time
Space first rises at swap 2. The swaps before it gain nothing you can use. Each swap is one rebuild of a 8TB array member: 11.1 hours at 200MB a second, 22.2 hours at 100MB a second.
| Drives swapped | Usable space | Gained | Rebuild time so far |
|---|---|---|---|
| 1 of 4 | 24TB | nothing yet | 11.1 hours to 22.2 hours |
| 2 of 4 | 32TB | +8TB | 22.2 hours to 44.4 hours |
| 3 of 4 | 40TB | +16TB | 33.3 hours to 2.8 days |
| 4 of 4 | 48TB | +24TB | 44.4 hours to 3.7 days |
The two rates are yours: read them from your NAS during a rebuild or a scrub. A system that rebuilds only the used space finishes sooner. The last step, expanding onto the new space, takes extra time this tool cannot estimate.
- Synology supports replacing drives to expand SHR, SHR-2, RAID 1, RAID 5, RAID 6, RAID 10 and RAID F1 pools.
- Replace the smallest drive first. When every drive is the same size, at least two have to be replaced with larger ones before the pool can grow.
Path 2: add drives to empty bays
All 4 bays are in use, so there is nowhere to add a drive.
Path 3: back up, start again with 4 x 16TB
A new array of 4 x 16TB in SHR (Synology Hybrid RAID) holds 48TB, the same as the end of path 1, reached in one build instead of 4 rebuilds. It needs a complete copy of your data somewhere else first, because creating the new array erases the drives. At today's cheapest 16TB drive, 4 drives cost $2,357.80.
16TB drives that fit, by price
New 3.5-inch SATA drives rated for NAS or enterprise use whose maker confirms CMR recording, no larger than this model's 24TB per-bay limit. Prices are live from Amazon.com. As an Amazon Associate the site earns from qualifying purchases.
The rules this tool applies, with their sources, are on its own page: NAS Upgrade Advisor.
Safe steps
These actions can lose data. Do none of them until a copy exists elsewhere:
- Replacing a drive when there is no spare bay. The pool sits in Degraded status until the repair with the new drive finishes. Synology says the expansion does not delete data but strongly recommends a backup first.
- Replacing more than one drive at a time. Synology allows only one at a time.
- Resetting the NAS to fit larger drives all at once. Synology's route creates a new pool, so your data comes back only from a backup.
- Expanding a pool that is not Healthy. Synology says to follow the Suggestion shown and resolve the issue first.
- Emptying a Recycle Bin. Files in it are not yet permanently deleted; emptying it removes them for good.
What to buy, if anything
Answer the questions under "Check your situation" and this section narrows to what your case calls for. Until then every table is shown, each with the case it is for.
NAS hard drives for an empty bay or a larger replacement
Why this is here: Only for readers whose space check shows the pool itself is full: these are candidates for an empty bay or for replacing the existing drives with larger ones. Match the drive type already in the pool (SATA HDD with SATA HDD). New 3.5-inch SATA NAS and enterprise hard drives whose maker confirms CMR, sorted by price per TB.
Sources
- Synology Knowledge Center: How do I expand the storage pool, dual pool, or volume capacity? checked 2026-10-14
- Synology DSM 7 help: View the Volume Usage checked 2026-10-14
- Synology Knowledge Center: Storage space is not freed up after deleting files or data. What should I do? checked 2026-10-14
- Synology Knowledge Center: I added new drives to my Synology NAS, but the available capacity didn't increase. What can I do? checked 2026-10-14
- Synology DSM 7 help: Add Drives to Expand the Storage Pool Capacity checked 2026-10-14
- Synology DSM 7 help: Replace a Drive checked 2026-10-14
- Synology DSM 7 help: Repair a Storage Pool checked 2026-10-14
- Synology Knowledge Center: Which Synology storage system models can I use with Synology Expansion Units? checked 2026-10-14
Facts on this page were reviewed on October 14, 2026. Prices and stock are from Amazon.com as of October 14, 2026.