ZFS/nexenta instance failed drive question

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http://hardforum.com – I have a 16x1TB server in RAIDZ3. I had a disk throw a write error, went ahead and replaced it. I am aware that the rebuild does take a long time, probably 5-7 days. We're on day 2, and we lost a second disk Question is, do I go ahead and replace it now, or let the first disk finish? I am inclined to replace it now, but I figure I'd get some thoughts first. (Hardware)