Last week, I suffered yet another disk failure in my NAS. I'm running 10x500GB disks on an OpenSolaris box in a raidz2 configuration. This allows me to survive the failure of two disks without losing any data. I'd already had a disk fail and was awaiting a replacement from WD when I suffered another complete failure and, at the same time, a 3rd disk effectively failed as it starting throwing up read errors on several sectors. This caused my 4TB zpool to disappear, "losing" all data stored on it. Bugger!
However, the third disk was not totally unusable; I was able to read from most of the disk. What I needed to do was copy as much as possible from the "bad" disk onto a new disk, put it back in the zpool and cross my fingers.