Let me add that you really want to have a hot spare, and if you can't do that (e.g. no extra bay in the chassis), then you should not delay on replacing a failed disk. Have a spare disk on hand, and take the time to replace it ASAP.

We lost everything on a FreeNAS system once because we were alerted to a failed-disk error, but decided to wait until the next week to replace it. But then a second disk failed, and we lost the storage pool. Lesson learned!

But this was an operator error, not a system problem. FreeNAS has been tremendously stable and reliable for us.