What to do if your Mac’s Fusion Drive fails (or shows signs of failure)

Apple created the Fusion Drive when large-capacity SSDs were unaffordable except to high-end professionals. A Fusion Drive pairs a large, low-speed hard drive (5,400 rpm) with a small-capacity, super-fast SSD, typically 24GB to 128GB. MacOS optimizes storage so that the most frequently accessed files are stored on the SSD.

You can run into trouble when the Fusion Drive’s SSD starts to falter, before it completely dies. While SSDs have no moving parts, they have an effective lifespan of a certain number of write operations per memory cell. Internal software manages “wear leveling” to make sure cells don’t stop allowing new writes prematurely by ensuring each cell gets an equivalent number of writes over time.

As I noted in a review of the drive-diagnostic software DriveDx, smaller-capacity SSDs used intensively wear faster than larger ones because they have fewer cells over which to average writes. That makes it more likely your Fusion Drive’s SSD will wear down sooner than a 1TB or larger SSD used as a single drive.

Apple’s drives have internal monitoring firmware that produces data in the industry-standard S.M.A.R.T. format, but macOS doesn’t alert you about error messages. For SSDs, this would be particularly useful, because the remaining lifetime of an SSD before errors occur can be estimated by the manufacturer’s specifications combined with the drive’s statistics on writes. With no moving parts, there’s more predictability about coming failures.

That’s why I recommend installing some form of drive-health monitoring software if you use a Fusion Drive. I recently reviewed DriveDx, but there are several other apps available, too. This category of software checks your drives on a schedule and alerts you the first time an error is noticed or a problem rises above a threshold. These reports can provide detail about the current wear state of your Fusion Drive SSD (or a regular full-disk one).

My colleague John recently wrote in and eventually ran four different disk diagnostics, because not all of them agreed initially. He has a 2015 iMac and is a heavy user of image-editing software, and the better diagnostic packages agreed that his SSD had already worn to near-failure status after just four years. For comparison, I have a 2017 iMac with a Fusion Drive (a 28GB SSD and 1TB hard drive), and DriveDx reports it’s only consumed about 10 percent of its estimated lifetime of use after two years of daily use.

mac911 ssd drivedx failed IDG

DriveDx’s diagnostic shows a very not good situation.

If your Fusion Drive’s SSD appears to be failing, ensure you have a complete backup of your Mac before you do anything else. Use Time Machine or any of several disk-cloning apps, like SuperDuper! or Carbon Copy Cloner.

What to do if a Fusion Drive’s SSD is failing

There are several options.