I'm running FreeNAS-8.0-RC2-amd64 and currently have one volume. This volume consists of 3 2TB disks in a Raid-Z format. I would like to expand this volume (not add a new one) and would like to know how to go about doing that. I've done some Googling but as I'm new to FreeNAS it still left me pretty confused. I have one new 3TB disk that I'd like to use to expand the volume, is this possible? Or would I need to add 2 3TB disks to expand the volume? If somebody could layout what options I have in terms of expanding my current volume or even point me to some resources I can go over that would be greatly appreciated. Thanks in advance for any help!

You can't expand a vdev like this. The only way to expand the pool is adding a new raidz vdev. tl;dr you need 3 more disks at once to expand the pool. PS: I would consider upgrading to 8.3.0, 8.0-RC2 is _really_ old and very buggy.

Thanks for the reply. So if I have 3 similar disks I can expand the pool? Otherwise I would just add another volume. So since I have one 3TB drive already, I would need 2 more of the same drive to expand the volume. Is this correct? Can I upgrade without losing my current volume?

Thats correct. Yes, but for 8.0 you have to save config, reinstall and upload config. This is to save configurations, the volume/pool is completely independent of freenas version and can be auto imported from any version.