Sorry for the long post, but you're a little mistaken;
actually, the SMART information on a disk contains certain threshold levels which predict whether a drive will fail. Of course this can't be determined with 100% certainty, but think of it more like "This drive will probably fail soon, so you should replace it."

The thresholds are programmed into the drive's firmware, so it's easy to see if a threshold has been reached. Every hard drive gets errors during its lifespan, but a lot of them can be corrected by the drive itself using checksums, spare sectors etc. As a drive gets older or begins to fail the error rate will increase, and this is detectable by querying the SMART info. According to a website I read about it about 60% of all hard drive failures can be predicted this way - That's a lot better than not getting a warning at all.

In reply to Re^6: SMART info from drives in win32 using WMI ('fail') by Nemurenai
in thread SMART info from drives in win32 using WMI by Nemurenai

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