There are vdevs (a collection of hard drives you assign together to mirror, have RAIDZ, RAIDZ2, etc.), and then there are zpools, a collection of 1 or more vdevs that get treated as a single "storage pool" like /mnt/tank. You make a zpool by combining drives into 1 or more groups of vdevs. Your "zdrive" doesn't have raidz2 in your example, your "vdev" has raidz2" and it's in a zpool.
In that situation you can either start a new vdev with 12 drives (which is really the upper limit of how big a vdev should be) and backup/copy the data over, or you can simply add another vdev of 6 drives into your zpool, so you'd have two vdevs, both with six drives, both with raidz2 and 2 drive parity. This is called
extending a pool.
A real life example: I started my build with a zpool containing one vdev with 4x4TB in raidz. I later upgraded and bought 4 more 8tb drives, which I made into a raidz vdev as well and then extended my pool. My pool has two vdevs, each with raidz (1 drive parity)
In terms of parity, if two 4TB drives were to fail in my pool, I'd lose the entire pool and data on both vdevs. If a 4TB and an 8TB fail, then I would not lose my pool. That's how a pool <-> vdev <-> hard drives works.
If you set up a single vdev of 12 drives in Raidz2, then 10 drives at 6TB would be available to you and you'd have 2 drive redundance in your single vdev of your total pool.
If you set up two vdevs of 6 drives in Raidz2, then 8 drives at 6TB would be available to you and you'd have 2 drive redundancy in each vdev of your total pool. You need to put this drive in its own pool instead (much like the SSD I use to store my jail webservers, which gets snapshots stored back in the main storage pool)
Lastly, I don't believe the zpool will stop you from doing something stupid, like add a vdev of a single drive with no parity to your pool. If you do this, and that drive fails, you will lose your entire pool.
There are plans to eventually get RaidZ extension support, which would allow you to add drives to a vdev without wiping it, but that's not yet available AFAIK.
Hope this explanation helps.
