What Is the Right Way to Expand TrueNAS Storage?
As an Amazon Associate we earn from qualifying purchases. Prices and availability come from Amazon.com as of the last update and may change.
It depends on your free bays. With one RAID-Z vdev and too few bays for a second vdev, TrueNAS 24.10 and later can extend that vdev one disk at a time. With room for a full new set, add a vdev that matches the first; with no free bays, replace every disk in the vdev with a larger one, then expand the pool. Check snapshots first, because deleting files does not free space a snapshot still holds.
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:
- Taking a disk offline to replace it in the same slot. TrueNAS says this reduces redundancy during the process, and to use an empty slot when you have one.
- Losing more disks in a vdev than its RAID-Z level allows. TrueNAS says the data in the pool is then lost, and that another disk can fail while one is resilvering.
- Replacing the next disk before the resilver finishes. TrueNAS says to wait for each resilver to complete.
- Starting a RAID-Z extension on a pool that is not healthy.
- The Force option destroys any data on the replacement disk, and TrueNAS wipes the replacement disk as part of the process.
- A Stripe pool has no redundancy. TrueNAS says a disk failure there means recreating the whole pool and restoring from backups.
- TrueNAS says RAID and disk redundancy are not a substitute for a backup.
- Back up the pool first. TrueNAS says RAID and disk redundancy do not replace a reliable backup strategy.[3]
- Confirm the pool is healthy before you start a RAID-Z extension.[1]
- Buy CMR drives. TrueNAS's hardware guide calls SMR drives a poor fit for ZFS, with slower writes and longer, less reliable resilvers. Ask the drive maker which a disk uses, or look up the model in the Is My Drive SMR tool.[4]
- Consider a hot spare. TrueNAS's hardware guide says one can be added to any pool and lets the pool rebuild itself if a primary drive fails.[4]
- Work out the result before buying: the Upgrade Advisor for an extension or disk swaps, the ZFS capacity calculator for a new vdev.
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.
CMR NAS drives for a TrueNAS pool, by price per TB
Why this is here: The disks for whichever expansion path the reader picks: one more disk at least as large as the vdev's smallest for a RAID-Z extension, a full matching set for a new vdev, or a larger disk for every disk in the vdev for a replace-and-expand. TrueNAS says SMR drives are a poor fit for ZFS, so only maker-confirmed CMR drives are listed. New 3.5-inch NAS and enterprise hard drives whose maker confirms CMR recording, sorted by price per TB on Amazon.com.
Sources
- TrueNAS 25.10 documentation: Managing Pools checked 2026-10-13
- TrueNAS 25.10 documentation: Replacing Disks checked 2026-10-14
- TrueNAS documentation: ZFS Primer checked 2026-10-14
- TrueNAS 25.10 documentation: TrueNAS Hardware Guide 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.