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.
I wanted to upgrade the iMac’s NVMe SSD with something that offered more space but was relatively less expensive (in other words, not sold by Apple). I found conflicting info about the upgrade, though. Authoritatively stated advice such as “You’ll kill the machine and possibly yourself” was hardly useful (not to mention, completely incorrect), while “Well, it worked in my 2017 [iMac]” provided hope, but no specifics.
Upgrading an older iMac’s PCIe SSD: Third-party solutions that save you beaucoup bucks was orginially posted by Jon L. Jacobi