Chosen Solution
I watched this disassembly video: https://www.youtube.com/watch?v=81PjGv8B… It looks like this 2015 iMac has both the flash storage and a hard drive, something I thought wasn’t offered. So I’m guessing it should be possible to add a HD drive to a flash storage CTO iMac and get the best of both worlds. One could presumably put in a SATA SSD, adapter bracket for a 2.5" form factor drive and maybe a digital thermal sensor cable from OWC.
Yes, you can add in a HD drive to a SSD only model. I must warn you this is not an easy task! The ‘Thin Series’ iMac’s are a bear to open and if you’ve not careful you can damage the display if you don’t follow the IFIXIT guide to the letter and use the correct tools. Here is the IFIXIT guide you’ll need to follow: iMac Intel 27" Retina 5K Display Hard Drive Replacement. This model uses a 3.5" drive you’ll need both a drive frame and the OWC inline thermal sensor. If you decide to put in a second SSD you’ll need an adapter frame to hold the 2.5" SSD in the 3.5" drive bay. Frankly, I recommend going with an external Thunderbolt 2 drive as the throughput is higher than the internal SATA III (6.0 Gb/s) of the HD port. Here’s one choice: OWC Elite Pro Dual
