Diving deeper into drive health: SMART utilities compared

There is continuing speculation over the real-world life of solid-state drives (SSDs), now fitted to all Apple’s laptops and (as of yesterday) the only type of drive supported by Apple’s new file system APFS. Although they have no mechanical components to wear out, their memory chips only accept a certain number of writes before they become likely to fail.

Calculations of expected SSD life seem very encouraging, but many of us have now heard of users whose SSDs have not survived three years – the period guaranteed by most hard disk manufacturers.

This article looks at how you can check the S.M.A.R.T. indicators on SSD and hard drives in, and connected to, your Mac so that you can keep an eye on their degradation, and try to prevent problems before they become serious.

S.M.A.R.T. indicators are not perfect predictors of SSD life, nor of drive failure. Studies performed in data centres with very large numbers of rotating drives have shown that some indicators are reliable, and when they start going wrong you should be worried, while others are seldom of much use.

I have previously explained how you can use indicators to monitor the ageing of SSDs. The key measurements here are S.M.A.R.T. indicator 173, the wear levelling count, and indicator 175, the total host writes.

macOS support for checking external drives has also been notoriously poor. Currently, as of Sierra, macOS only allows the checking of S.M.A.R.T. indicators on internal and Thunderbolt drives; those connected to a USB (including USB3) port cannot be checked without installing third-party software support. Those in NAS and external hardware RAID arrays depend entirely on the external system’s support for access to S.M.A.R.T. indicators.

It remains a mystery as to why Apple continues to ship incomplete support for S.M.A.R.T., particularly as its retail and online stores are only too happy to sell customers USB storage, and to benefit from those profits. I suspect that they would have paid for the cost of development many times over.



There are two ways of checking S.M.A.R.T. status and other important information about storage devices in macOS: Disk Utility and System Information. Although both can provide quite a lot of other information about internal and external drives, the best that they can do here is state whether S.M.A.R.T. status is “verified” or failing. Neither utility tells you when that status was last checked, either.



This £5.99 utility from the App Store improves on macOS by stating when S.M.A.R.T. status was last checked, and is designed to sit in the background, making periodic checks and alerting you to any problems as they develop. It also checks for infamous I/O errors, which are another indicator of incipient drive problems.


My main problem with this app is that it is designed to be left running, monitoring status, and doesn’t behave like a normal app. It also doesn’t dive any deeper into detail, which might enable you to watch your SSD’s graceful degradation with use.

TechTool Pro

This rich array of hardware testing tools includes S.M.A.R.T. status checks, which can be viewed from a single test, or monitored in the background. Rather than reveal the individual results, it displays a set of twelve (for SSDs, more for hard drives) indicators, on an analogue scale from Pass to Fail.


In some cases, this simple scale has untoward effects. Indicator 194, Internal Temperature, seems to pass when it is coldest, when a Mac has only just been started up, and much of the time sits in the supposedly worrying gap between pass and fail. This scale is not appropriate for this indicator.

Drive Scope

Micromat, vendors of TechTool Pro, have recently launched this new tool, which dives more deeply into S.M.A.R.T. indicators, and reveals their measured values. This appears immature at present: for example, indicator 169, which TechTool Pro states is ‘Total Bad Block’ (Count), is here simply described as ‘Unknown’.


The key metrics of SSD life, indicators 173 and 174, are also presented in an unhelpful way: the raw value of the Wear Levelling Count is a large integer, its corrected value is then, for example, 182, against a stated threshold of 100. Making sense of those numbers is not easy. However, Drive Scope is a valuable improvement over TechTool Pro, and these wrinkles should iron out quickly.


I have been a fan of DriveDx for a long while now. It remains the only comprehensive drive tool, providing full information and S.M.A.R.T. indicators for all drives which are supported by macOS. As with Drive Scope, it can also cover USB drives if you install a free extension.


For an SSD, DriveDx gives all twelve S.M.A.R.T. indicators, with raw values, descriptions of what they mean, changes, and more. Its interpretation of drive temperature is still a little puzzling, but for indicators 173 and 174, it is detailed and very helpful. If you want to monitor the ageing of your SSD, this is exactly what you need.


DriveDx is starting to get some competition from Micromat’s Drive Scope now, but remains head and shoulders the best tool for assessing drive health. It’s available from Binary Fruit for $24.99, but is currently on offer for $19.99.

If you have just installed High Sierra onto your SSD, or are intending to do so, you might find DriveDx very useful.