This diskfailure should not panic a system, but just disconnect disk from ZFS

[Quartz]

Also:

> And thus I'd would have expected that ZFS would disconnect /dev/da0 and
> then switch to DEGRADED state and continue, letting the operator fix the
> broken disk.

> Next question to answer is why this WD RED on:

> got hung, and nothing for this shows in SMART....

You have a raidz2, which means THREE disks need to go down before the 
pool is unwritable. The problem is most likely your controller or power 
supply, not your disks.

Also2: don't rely too much on SMART for determining drive health. Google 
released a paper a few years ago revealing that half of all drives die 
without reporting SMART errors.

http://research.google.com/archive/disk_failures.pdf