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.
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.
Replace the Fusion Drive’s SSD. Depending on your Mac model and year, it may be relatively simple, fiendishly difficult, or even impossible to swap out the dying or dead SSD. If you do it yourself, the cost is your time plus a relatively affordable part—but you have to factor in the potential that you might damage your Mac, especially probable with an iMac, which requires removing the screen. iFixit has guides by model, so take a hard look at the steps, time, and tools involved.
Swap the Fusion Drive’s hard drive with an SSD and stop using the Fusion Drive’s SSD. If you’re willing to go to all the trouble to crack open your Mac, the price of 512GB and 1TB SSDs has dropped so much that it makes more sense to replace the hard drive with an SSD that comes in the same form factor. Other World Computing and other companies sell SSDs in hard drive packaging; OWC has guides for how to figure out which ones are the right choices.
Switch to an external SSD. With USB 3 (Type A connector), USB 3.1 (Gen 1, SuperSpeed at 5Gbps, or Gen 2, SuperSpeed+ at 10Gbps) over USB-C, or Thunderbolt 3, you can get enough throughput from an external SSD to use that as your startup volume and main volume. This is a great option if you don’t want to or can’t open your Mac.
Depending on your configuration, you might purchase a 512GB external SSD and use it for macOS and documents and files you commonly work with. Reserve your existing internal Fusion Drive HDD of 1TB to 3TB for less-intensively used files, like your Photos Library and Music Library.
This Mac 911 article is in response to a question submitted by Macworld reader John.
Ask Mac 911
We’ve compiled a list of the questions we get asked most frequently along with answers and links to columns: read our super FAQ to see if your question is covered. If not, we’re always looking for new problems to solve! Email yours to firstname.lastname@example.org including screen captures as appropriate, and whether you want your full name used. Not every question will be answered, we don’t reply to email, and we cannot provide direct troubleshooting advice.
When you purchase through links in our articles, we may earn a small commission. This doesn't affect our editorial independence.