I have installed TrueNAS-SCALE-24.04.2.3 on a 3 disk server computer. The main disk is a 120 GB SSD. The other 2 disks are 1TB HDD each. I whant to install all the software in the OS disk (120GB SSD) and the other two just for data storage (zfs pool as 1 x 2TB disk with no raid). When installing an app, which disk does it use?
It uses whatever pool you tell it to use in your app configuration, which can be anything other than the boot pool. Using the boot device for anything other than the boot pool is not supported, as both the docs and the installer tell you.