Thank you SO MUCH for this guide. I bought my iMac used from someone who was upgrading to a newer SSD model, and once I got it home and reset, I was understood all too well why he upgraded. So I ordered the kit here and followed these directions. With my iPad propped up next to the iMac on my table, I was able to follow it right through. As others have commented, the drive cable is very short, but I held back the rubber bumpers until I got it connected, and then everything else went smoothly. Now my iMac is like a new machine (minus the newer Thunderbolt ports and other newer amenities). Thanks again!
The step-by-step guide was awesome and easy to follow. As others have commented, getting the SATA connector onto the SSD is tricky but not impossible. Just pull back the rubber bumpers long enough to reach the connector, then flip them back into place before seating the drive. One comment for my machine (late 2015 21.5”): There is no mic hole for my model, so while I was a little confused at the instructions on reinstalling the adhesive, the drive replacement went like a dream. And I saved myself the $120 labor the local Mac shop wanted to charge me.