It is 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.

Select Storage.

• Navigate to the pool.
• Select the gear icon
• Select Add Vdevs.

• Add the hard drives to the second Vdev.
• Select Add VDEVS.
  • Depending on the RAID type being used, it will automatically be selected.

This will prompt for confirmation:

• Select Confirm.
• Select Add Vdev.

• View the Pool Status.