In 2007 or so Google did a study called Failure Trends In a Large Disk Drive Population. They found that 36% of the drives in their study failed suddenly without any SMART warning at all.
The abstract notes the following:
Google Study wrote:Our analysis identifies several parameters from the drive’s self monitoring facility (SMART) that correlate highly with failures. Despite this high correlation, we conclude that models based on SMART parameters alone are unlikely to be useful for predicting individual drive failures. Surprisingly, we found that temperature and activity levels were much less correlated with drive failures than previously reported.
Section 3.5.6 of the report states:
Google Study wrote:Figure 14 shows that even when we add all remaining SMART parameters (except temperature) we still find that over 36% of all failed drives had zero counts on all variables. This population includes seek error rates, which we have observed to be widespread in our population (>72% of our drives have it) which further reduces the sample size of drives without any errors.
It is difficult to add temperature to this analysis since despite it being reported as part of SMART there are no crisp thresholds that directly indicate errors. However, if we arbitrarily assume that spending more than 50% of the observed time above 40C is an indication of possible problem, and add those drives to the set of predictable failures, we still are left with about 36% of all drives with no failure signals at all.
My emphasis. Get those backups done!