My second HD replacement for this iMac
The hard drive was failing. Over time (after about 5 years), the boot time began to increase and everything just got slower. Weird things began to happen and I thought I had an infected (virus?) HD. This drive was actually a replacement of the original 500GB HDD that shipped with this 27" iMac. I had two partitions on this 1TB HDD so I could boot into either El Capitan or High Siera. But then I couldn't boot at all or it took a long time (20 -30 min.). Even after trying to reformat and restore everything, it just wasn't working right. Time to replace the HDD again, only this time with a 2.5" SSD, hence the ifixit drive adapter kit. I could have gotten a much cheaper drive adapter, but fixit's kit had the needed SSD temperature sensor and cables (Thanks!).
Having used fixit's repair guide to do the original iMac drive replacement, and their awesome guides for other Macs and cameras I had to repair, it is a no brainer to go here for this replacement again. The repair went very smoothly up until the point where I had to reconnect the display vertical sync cable. As described in the guide, this cable in my case was a very fragile ribbon cable. After three attempts to get this connected right, I succeeded. See my advice for this cable.
Though the 3.5" HDD to 2.5" SSD replacement went fairly smoothly (I didn't even have to completely remove the display to do the HD replacement), heed the warning about the vertical display cable. This is the first cable you will need to remove after you first lift the display from the case. In my case, it was a thin fragile ribbon cable. I fail to see why such a fragile cable is used here... and with a connection that requires some force to reconnect. This cable has four very thin and very fragile wafers of copper NOT glued to the backing plate that supports them! If you don't have them completely flat on the backing, you will end up with bent wafers that won't go into the connector!
My advice: use small needle nose pliers (without serrations), to flatten the pins completely against the backing plate. Start from the innermost exposed area and work your way out to the tip of the connector to ensure complete flatness of all four wafers.