Yes, you can add a single disk to an existing pool in TrueNAS. Since TrueNAS 24.10 (Electric Eel, October 2024), the Extend button on a RAIDZ vdev widens it by one disk at a time, so a four-disk RAIDZ1 can become a five-disk RAIDZ1 without a second vdev or a rebuild.
Most of what comes up for “TrueNAS add disk to existing pool” still says you have to add a whole vdev, which is out of date for RAIDZ. TrueNAS CORE never got RAIDZ expansion, so a CORE pool grows by a vdev or larger disks. The labels below are from TrueNAS Community Edition 25.10.7 (called TrueNAS SCALE before 25.04), the current stable release as of October 2026.
Whatever you pick, back up anything you can’t lose before you change a pool.
Which Way to Add a Disk Depends on Your Vdev Layout
A TrueNAS pool is built from one or more vdevs, and the redundancy lives inside each vdev (mirror, RAIDZ1, RAIDZ2 or RAIDZ3) rather than across the pool. ZFS builds that RAID in, unlike LVM. To check yours, open Storage and select View VDEVs on the pool’s VDEVs widget (Manage Devices on Topology in 24.10).
- RAIDZ1, RAIDZ2 or RAIDZ3: Extend the vdev by one disk, as often as you like.
- Mirror or single disk: Extend adds the disk as another copy, so redundancy, not space.
- Any layout, with enough disks: add a matching new vdev (permanent on a RAIDZ pool).
- Any layout, out of bays: replace every disk with a larger one, then expand.
None of these change a RAIDZ vdev’s parity level, so a four-disk RAIDZ2 extended to six disks still can’t lose more than two at once (the guide to setting up TrueNAS Community Edition and creating your first pool explains each layout).
Expansion is one of the four things I weigh when picking a vdev layout. In my January 2026 video on common TrueNAS mistakes, I said it isn’t as big of a deal as it used to be now that ZFS expansion exists. My own pool in that video was one vdev of eight 16 TB drives in RAIDZ2. An 8-drive RAIDZ1 vdev really doesn’t make a lot of sense unless it’s for something like an off-site backup where you have a full local copy (even then, you’re probably better off with RAIDZ2).
Extend makes it easy to keep widening a RAIDZ1, and the problem is the resilver. One of my eight drives failed. With RAIDZ1 I would have been pretty stressed out, even with backups, because one more failure while the pool was rebuilding would lose everything. With RAIDZ2, another drive can die without losing any data. If your RAIDZ1 is getting that wide, add a matching second vdev or recreate the pool as RAIDZ2 (more on which RAID/vdev type is right for a NAS).
TrueNAS: Add Disk to Pool With RAIDZ Expansion (Extend)
Extend widens a RAIDZ1, RAIDZ2 or RAIDZ3 vdev by one disk in place. It’s RAIDZ expansion, part of OpenZFS 2.3 (January 2025), which TrueNAS shipped a few months early in 24.10, and OpenZFS’s own documentation covers how it works.
The new disk has to be at least as large as the smallest disk in the vdev, and the pool should be healthy. It also needs the raidz_expansion feature flag, so a pool created on an older release may need Upgrade on the Storage Dashboard first, which can’t be undone (no rolling back to an earlier major version). Extend can’t be undone either.
1. Install the new disk, then open Storage and check that the pool shows as healthy.
2. Select View VDEVs on the pool’s VDEVs widget.
3. Select the RAIDZ vdev itself (the vdev row, not one of its disks), then select Extend to open the Extend Vdev window.
4. Pick the new disk from the New Disk dropdown and select Extend. A job progress window opens, and TrueNAS returns to the VDEVs screen when it’s done.
5. Repeat for each further disk, one at a time.
How Long It Takes and What Happens Meanwhile
Expansion reads the data on the vdev and rewrites it onto the new layout, so a full vdev takes longer than a nearly empty one. The pool stays accessible with its redundancy intact. If the system restarts, the expansion resumes where it left off. If a disk fails mid-expansion, it pauses until the vdev is healthy again, and when it finishes, OpenZFS scrubs the pool to verify every block it copied.
Why the Pool Shows Less Space Than Expected
Existing data keeps its old data-to-parity ratio, and only new writes use the wider layout, so the old data takes more room than it would on a freshly built vdev. You get that space back as data is modified or deleted, or sooner if you replicate and rewrite it. The capacity TrueNAS reports keeps using the old ratio even then, a gap TrueNAS says doesn’t affect the space you can actually use.
How ZFS compares with Btrfs covers the other filesystem people weigh it against.
Add a Disk to a Mirror or a Single-Disk Pool
Select the mirror on the VDEVs screen, select Extend, pick the disk from New Disk, and select Extend. TrueNAS attaches it as another copy and resilvers it, so a two-way mirror becomes a three-way mirror. That’s redundancy, not space.
A single-disk pool is a stripe vdev with no redundancy, and selecting that disk and then Extend attaches the second disk as a mirror. That’s the upgrade path if you started with one disk, but it’s still one disk’s worth of space.
To get more space on a mirror pool, add another mirror pair as a new vdev (the next section), keeping in mind that mirrors cost you half the available storage space.
TrueNAS: Expand Pool by Adding a New Vdev
Adding a vdev expands a pool by several disks at once, and short of replacing every disk, it’s the only way to add space to a mirror pool. The new data vdev has to be the same type as the existing ones and should match their width. In my April 2025 setup guide video I put six drives in two RAIDZ1 vdevs (“I want the width of each vdev to be three total disks”). A third vdev would take three more disks, against one for Extend.
On a RAIDZ pool, adding a vdev is permanent. A data vdev can only be removed (Remove on its ZFS Info widget) when every top-level vdev is a mirror or a stripe, so a RAIDZ pool can’t take one back out. Every data vdev is also a point of failure, so losing any one vdev loses the pool.
1. Install the disks, open Storage, select View VDEVs on the pool’s VDEVs widget, and then select Add VDEV. (Or select Add To Pool on the Unassigned Disks widget, choose Existing Pool, pick the pool and select Add Disks.)
2. In the Add VDEVs to Pool wizard, select Next to reach the Data step, where the layout is already locked to match your existing vdev. Use Automated Disk Selection to set the disk size, Width and number of vdevs, or Manual Disk Selection to pick the disks yourself.
3. Select Save And Go To Review, check the layout, and select Update Pool.
The same wizard adds spare, log (SLOG) and cache (L2ARC) vdevs.
Replace Every Disk With a Larger One
If you’re out of bays, swap each disk for a bigger one. A vdev is limited by its smallest disk, so the space only shows up once the last one is replaced, and TrueNAS doesn’t expand the pool automatically. It’s worth checking SMART results before you replace a drive, too.
1. On the VDEVs screen, select a disk. With no free bay, select Offline on its ZFS Info widget and swap in the larger drive, which drops the vdev’s redundancy by one disk (to none on RAIDZ1 or a two-way mirror) until the resilver ends. Then select Replace on Disk Info, choose the new disk from Member Disk, and select Replace Disk.
2. Wait for the resilver to finish (this can take a long time on a full pool), then repeat for every disk in the vdev.
3. On the Storage Dashboard, select Expand next to Export/Disconnect, then Confirm and Continue.
Rebuild the Pool With More Disks
Destroying the pool and creating it again is the only way to change a RAIDZ vdev’s parity level (RAIDZ1 to RAIDZ2, for example). It deletes everything on the pool. ZFS will never be as loose as Unraid about mismatched drives, but Extend makes this a much rarer step.
1. Back up the pool, check that the backup restores, and note its shares, apps and data protection tasks.
2. On the Storage Dashboard, select Export/Disconnect and tick Destroy data on this pool? Delete saved configurations from TrueNAS? is ticked by default and removes those too. Tick Confirm Export/Disconnect, type the pool name, and select Export/Disconnect.
3. Select Create Pool, add every disk with the layout you want, finish the wizard, and restore your data from the backup.
Final Thoughts on Adding Disks in TrueNAS
Overall, for most people with a single RAIDZ vdev, Extend is the answer now, as long as the vdev isn’t getting so wide that RAIDZ1 stops making sense. ZFS and TrueNAS change from release to release, and nothing here is guaranteed.
