How to Add a Disk to an Existing Pool in TrueNAS

  • Post author:WunderTech
  • Post published:January 6, 2023
  • Post last modified:November 30, 2023
  • Post category:TrueNAS
  • Reading time:8 mins read

In this tutorial, we will look at how to add a disk to an existing pool in TrueNAS.

Before we get started, I want to be clear about one thing right off the bat: if you are attempting to add an individual drive to a ZFS pool, you cannot.

adding a single hard drive in truenas. How to Add a Disk to an Existing Pool in TrueNAS

ZFS is entirely different than other filesystems and adding drives isn’t as simple as it is on other NAS operating systems. For example, adding a drive to Unraid is as simple as adding it and making a few configuration changes, whereas TrueNAS (depending on the scenario) is a lot more difficult.

With TrueNAS, you have two real options and we’ll take a look at both below. The first is to add the same number of drives that you currently have. Meaning if you have four drives in a RAIDz1 configuration, you can add another four drives and add a Vdev.

The other (not ideal) option, assuming you have backups of your data is to destroy the pool, then recreate it with the additional drive(s). We will look at both options below.

How to Add a Disk to an Existing Pool in TrueNAS

As mentioned above, if you’re attempting to add an individual drive to an existing ZFS pool, you cannot. We will look at how to add a Vdev, as well as how to destroy a pool and recreate it with the new number of drives (assuming you have backups). Before we do, I want to be clear on one final item:

If your goal is to simply add storage space, you can replace each drive one at a time. Meaning that if you have four, 4TB hard drives using RAIDz1, replace one hard drive with a larger size, wait for it to re-silver, then replace the second hard drive, etc.

You’ll have to do this for all hard drives but will increase the total storage size. Just keep in mind that the process will be slow, but it can be done.

Step 1: Creating a Second Vdev

If you plan on adding more hard drives to TrueNAS, it’s generally recommended to add the same number of hard drives that the pool currently has. Meaning that if you have four hard drives in your pool using RAIDz1, you can add four additional drives, create a Vdev and you will have two Vdevs that are both utilizing RAIDz1. TrueNAS will also automatically balance the data between both Vdevs.

1. Select Storage, navigate to your pool and select the gear icon, then Add Vdevs.

storage pool actions in truenas.

2. Add the hard drives to the second Vdev, then select Add Vdevs. Depending on the RAID type you’re using, it will automatically be selected.

adding a new vdev in truenas.

3. If you’d like to continue creating the Vdev, select Confirm, then Add Vdev.

vdev creation in truenas.

4. If you view the Pool Status, you’ll see both Vdevs!

pool status in truenas.

Step 2: Recreating the Storage Pool

Before proceeding, make sure you have backups of all important data. While not ideal, one way that you can add the new drives without having to add the same number of hard drives currently being utilizes is by destroying and recreating the pool. This is something that can work and while it’s not ideal, it’s sometimes the best option. However, I’d explore all other options before completing this. The OS being used is TrueNAS Scale but it will be very similar in TrueNAS Core.

1. Select System Settings, then under System Dataset Pool, change this to your boot pool, and save.

changing the pool to the boot pool.

2. If you’re absolutely positive you’d like to proceed, select Confirm, then Continue and Save.

confirming that the smb service will be stopped.

3. Select Storage, then select the gear icon next to your TrueNAS Pool and select Export/Disconnect. In the dialogue, ensure that all options are selected (once again, confirming that you want to delete your data), then select Export/Disconnect. It will take a few minutes to delete your pool.

disconnecting a storage pool in truenas.

4. Select Create Pool after the pool has been deleted, add all of your hard drives, select the RAID type you’d like to use, and then select Create.

Please keep in mind that you’re going to experience the exact same issue if you attempt to add an individual drive again at a later time.

creating storage pool in truenas scale.

Conclusion & Final Thoughts

This tutorial looked at how to add a disk to an existing pool in TrueNAS. There are two real ways that you can expand a pool, with simply replacing the existing hard drives with larger hard drives being a legitimate option as well.

The option you should use will depend on the scenario that you have, but you must keep in mind that ZFS does not operate the way that a filesystem like Btrfs does. Simply “adding a drive” isn’t going to work, as ZFS is really designed for adding multiple drives, if necessary.

Thanks for checking out the tutorial on how to add a disk to an existing pool in TrueNAS. If you have any questions, please leave them in the comments!


Frank is an IT professional with 13+ years experience and the creator of WunderTech. He focuses on sharing his experience with others on computer hardware, servers, software, networking, and self-hosted apps. He has a BS in Computer Information Systems and an MBA. Learn more about Frank in his bio.