You frequently see RAID cascade failures. Drive A fails. You pop it out, put in a new disk. Machine starts rebuilding RAID array. This involves a ton of reads

You frequently see RAID cascade failures. Drive A fails. You pop it out, put in a new disk. Machine starts rebuilding RAID array. This involves a ton of reads from other disks. Under the increased strain, Drive B, which was already on the edge, also fails. And so on. I'm no longer a believer in redundant disks. Redundant machines would be better.