Date: 09-08-10 02:04
You can add HDD to a pool, but you cannot go from raidz1 to raidz2 for example.
It is not possible to add a disk as a column to a RAID-Z, RAID-Z2, or RAID-Z3 vdev. This feature depends on the block pointer rewrite functionality due to be added soon. You can however create a new RAID-Z vdev and add it to the zpool.
I should explain this in more detail. In short, I had to create a 7 vdev array with only 5 physical HDD.