Thanks for all the very helpful information. I am thinking about taking the plunge and opening my late 2015 27” 5K retina iMac (17,1) to swap out both HDD for a 2 GB SATA SSD, and the currently *dead* blade SSD for a new (OWC) 1 TB SSD. Seems risky to spend about $500 for all the components and parts to upgrade a 5 1/2 year old machine given that it may not work (or I might screw it up). But the 5K retina screen on this machine is still top notch and would love to get a few more years out of it. Your success stories @jorge and @thanhtrdang are encouraging!