From <a href="http://labs.google.com/papers/disk_failures.pdf" rel="nofollow">http://labs.google.com/papers/disk_failures.pdf</a><p>"Out of all failed drives, over 56% of them have no count in any of the four strong SMART signals, namely scan errors, reallocation count, offline reallocation, and probational count."<p>Most drives that fail have no SMART errors. In other words they fail spontaneously. BTW, if you have a drive with even a single error, replace it now or at least make sure you have a spare on hand:<p>"After the first scan error, drives are 39 times more likely to fail within 60 days than drives without scan errors."<p>"After their first reallocation, drives are over 14 times more likely to fail within 60 days than drives without reallocation counts, making the critical threshold for this parameter also one."