Apple relied on the Fusion drive for years as an affordable middle option between a slow but affordable high-capacity hard disk drive and a fast but expensive high-capacity SSD. The Fusion drive pairs an SSD with very few gigabytes with a much larger HDD. macOS treats the Fusion drive as one logical device, displaying it as a single drive in the Finder. But behind the scenes, it automatically optimizes file storage, so the most frequently accessed files are shifted over time to your SSD, while the HDD holds less-used ones. This is why your Fusion drive Mac can start up quickly, but still be a slug when using Photoshop to edit large images.
This seamless and invisible management is nothing like having two separate drives—it’s a stitched-together hybrid. As a result, you can’t separate the two just by upgrading one part without a lot of fancy interactions. And most Macs with Fusion drives have an SSD installed that is difficult to access or is build into the motherboard; the HDD is often quite difficult to access and swap out, or may be impossible to remove.
If your plan is to change out either or both parts, however, start by cloning your Fusion drive to an external drive using Disk Utility, Shirt Pocket’s SuperDuper, or Bombich Software’s Carbon Copy Cloner with Catalina or earlier versions of macOS. Big Sur adds complexity to cloning; read this post at Bombich Software and this at Shirt Pocket for more on the issue. (Macs with Apple’s M1 silicon can’t be cloned fully yet, but no M1 Mac comes with a Fusion drive.)
Apple offers a detailed guide on splitting and restoring Fusion drives that should help if you want to knit back together one or two upgraded components.
This Mac 911 article is in response to a question submitted by Macworld reader John.
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