Artie is dead on. OnyX, Disk Utility, Cocktail, TinkerTool System, and a host of other utilities all use the same Unix command to perform a SMART check. It reports only an aggregate pass/fail. TechTool Pro actually talks to the drive itself and reports all of the several SMART parameters reported by the disk drive along with information on what the pass/fail value is for each SMART parameter.

TechTool Pro 8 shows 23 different SMART parameters reported by the HDD portion of the Fusion drive on my Mac mini and 10 SMART parameters for the SSD portion. The HGST drives in my RAID array report 19 SMART parameters but many are different parameters than are reported for either my other HDD or SSD. It all depends on what parameters the vendor collects and what pass fail limits the vendor sets for each.

The Google Labs test of HDD failures indicated SMART was a poor predictor of impending drive failure because the manufacturers set the SMART limits too high. SMART is better at telling the user the drive has already failed. Google Labs recommends the use of surface scans as the best predictor of impending drive failure. As I said previously it is the presence of NEW bad data blocks that is a reliable indicator the magnetic surface media on the drive is beginning to fail and flake off.

Last edited by joemikeb; 01/27/16 12:24 AM. Reason: I hit post too soon
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