"Well let's put aside the disk failure issue. Modern 4-drive NAS systems can tolerate a lost drive and alert you promptly to replace it. You'd have to have pretty bad luck to lose more than one drive at the exact same time."

Very interesting and well thought out posting, but the above quotation represents a very, very naive understanding of how these arrays work with large, multi-TB hard drives.

In fact, the reality is exactly backwards to what he has written here: with multiple terabytes of data on the array, a single drive failure results in a long, intensive rebuild process that can serve to hasten the failure of the remaining drives.

I am not anti-NAS - I use them myself for critical data - but with 3 and 4TB hard drive, I would only use raidz3 (or equivalent) at this point (and preferably with 12 or fewer drives in the array).