If you bought entry-level iMac with at least a Fusion Drive, here’s how to upgrade to something faster, more capacious—and cheap (relatively).

I love Retina iMacs. So much so that I took a chance and bought a used 2015 low-end Core i5 iMac with a 1TB Fusion Drive (which combines a 24GB NVMe SSD and a 1TB hard drive)—a machine that’s actually slower than my 2012 Core i7 iMac. I bought it with upgrading in mind, which is where the fickle finger of fate came into play.
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https://www.pcworld.com/article/3386117/upgrading-an-older-imacs-pcie-ssd-third-party-solutions-that-save-you-beaucoup-bucks.html