What are the chances for failure of 2 drives at the same time on a 4 drive raidz 1? I've read that chances are 1 from 10 drives to fail over a period of time of one year or more. What is the best approach to having more space but still have redundancy? 3 drives in raidz 1? 6 drives in raidz 2? I would like to use a pool of 4 x 1.5 TB drives for a mini backup server. I've read some posts regarding ZFS here on forum and on http://www.solarisinternals.com/ but I need your opinion on that: chances of failure for hard drives from your experience.