How do you currently have disks organized?
Never mind, I see...
You know, you could also create a new RAIDz vdev with 3 disks and add it to the current zpool. You would loose 2TB, but then again, RAIDZ arrays bigger than 5 disks are not recommended because of the rebuild time. It's better to have many small RAIDZ vdevs in one pool because of better security. If you have a large vdev, it takes a long time to rebuild and there are grater chances another drive will crash.
Matej
Never mind, I see...
You know, you could also create a new RAIDz vdev with 3 disks and add it to the current zpool. You would loose 2TB, but then again, RAIDZ arrays bigger than 5 disks are not recommended because of the rebuild time. It's better to have many small RAIDZ vdevs in one pool because of better security. If you have a large vdev, it takes a long time to rebuild and there are grater chances another drive will crash.
Matej