Like so many of the high-tech devices that we use, storage drives have extensive monitoring systems to check that they are functioning properly. And just like your car, domestic heating system, and so on, almost all the information from those monitoring systems is kept a closely guarded secret. That works well for most systems – many drivers seem unable to cope with even the limited display of a typical dashboard – but falls short for computers.

Disk Utility’s crude assurance that a drive’s S.M.A.R.T. status is “Verified” can cover a multitude of concerns. It does not, in many cases, do that simple task appropriately. Fusion Drives, consisting of a solid-state drive (SSD) and a hard drive, are given a single status report for the pair, so if there is a worrying result, Disk Utility does not reveal which of the drives you should be concerned about.

A few tools go further. For some years, third-party products such as SMARTReporter and SMART Utility have provided more detailed breakdowns of many of the variables actually recorded by S.M.A.R.T. monitoring, and general disk utilities such as Drive Genius can extend those if you wish.

Neither does OS X provide any support for monitoring S.M.A.R.T. results on most external drives, such as those connected via USB (even USB 3) or FireWire, although it does for those using more sophisticated buses such as Thunderbolt and eSATA.

DriveDx (by BinaryFruit, around £18.99) is fairly expensive compared with these other options, but is the most complete and extensive drive checking and monitoring tool available for OS X. What is more, it comes with uniquely helpful interpretive advice, which helps you work out what each of its gamut of figures means.

It works with almost all hard disks, SSDs, and Fusion Drives which are connected directly to your Mac. It does not (currently) work with hardware RAID systems such as my Promise Pegasus R4, which come with their own utility to assess the health of their drives, nor with USB memory sticks. If you want it to see inside USB or FireWire drives, you will need to download and install a free kernel extension (which might be interesting in these days of SIP) to open them up. You can leave DriveDx running from your menu bar, checking S.M.A.R.T. measurements regularly, or just use it when you wish.

On a hard disk, including that built into a Fusion Drive, it is hard to think of any information or measurement which DriveDx does not report. Its front page gives an excellent summary of the most important details, with clear indicators of the headlines. Some of these are very useful figures for other issues: for instance, the Power On Cycles Count shown confirms that, whatever my Energy Saver settings said, my iMac was putting its hard drive to sleep, by powering it down, when I had it set to keep the drive spinning.

The detailed health indicators are not just for nerds, though. Hover over, or select, any of the long list of measurements and there is an excellent explanation which informs you of what is being measured, how it is considered to be important, the significance of abnormal values, and other related indicators. You can also opt to have that information automatically updated. I cannot recall another technical tool being so helpful and explanatory.

The front page for an SSD within a Fusion Drive reveals almost everything you could want to know about it, again with the key points made crystal clear. My only slight concern here is that the SSD Lifetime Left Indicator is potentially misleading to the average user. This does not decline linearly, so the fact that my SSD had already fallen to 94% in slightly less than three months does not imply that it will fall to 40% in around two years’ time.

In truth, I think that the Lifetime Left Indicator is based on the standard S.M.A.R.T. indicator 173, Wear Leveling Count, which normally falls quite sharply to around 95% in the first weeks of use, and then remains almost static.

This is explained in more detail in the full health indicators for the SSD.

DriveDx is one of the most important utilities available for Macs. Even if you only use a single built-in SSD, it provides important insights which could save your data from creeping drive failure, and will stop you from worrying about the insignificant. Sadly, although abnormal S.M.A.R.T. results are quite reliable indicators of imminent drive failure, many hard drives still die out of the blue: nothing, apart from good backups, can protect you from that.

Buy it. Use it. It is worth as much as the data stored on your drives.

Postscript:

DriveDx is not longer available in the App Store, but is sold direct from Binary Fruit’s website.