The TrueNAS installer doesn’t have a way to use anything less than the full device. This is a waste of capacity when installing to large NVMe which is usually several hundred of GB or even TB. TrueNAS SCALE will use only a few GB for its system files so installing to a 16GB partition is sufficient.
This guide covers for TrueNas Scale 24.04 to 25.10
You can to modify the installer script before starting the installation process.
- Boot TrueNAS Scale installer from USB/ISO
- Select shell in the first menu (instead of installing)
- While in the shell, find and open the installer and edit using vi

