Hard drive(s) goes inside VDevs.
Vdevs go inside zpools.
Zpools store your data.
Disk failure isn’t the concern with ZFS. Vdev failure is! Keep the VDevs healthy and your data is safe
You cannot add more hard drives to a VDev once it is created
If any VDev in a zpool is failed, you will lose the entire zpool with no chance of partial recovery
Once you add a VDev to a zpool it cannot be removed for any reason. Even if you “just” added it because of a typo at the command line or a mistake in the UI
FreeNAS works great in a virtual machine such as VMWare ESXi, VMWare Workstation and VirtualBox if you want to experiment without dedicating hardware. Do not trust these with real data however!
Please do not run FreeNAS in production as a Virtual Machine!
ZFS has very few “recovery tools” unlike many other file systems. For this reason, backups are very important. If the zpool becomes unmountable and cannot be repaired there are no easy software tools or reasonably priced recovery specialists you can use to recover your data. This is because ZFS is enterprise-class software, and no enterprise would waste their time with recovery tools or data recovery specialists. They would simply recover from a known good backup or mirror server.