Adding another hard drive besides the SDD
I bought a unit with 256GB SSD pre-installed and I opened it and confirmed Apple actually left the 2.5" HDD cradle empty. It's actually using the SSD slot. I am thinking of adding another HDD to that cradle but its missing the SATA cables. Anyone can confirm is the cable connector the same as the 2011 models?
Sorry to say I don't think many people have dug that far into these newer systems yet.
With that said the IFIXIT teardown offers enough visual information to support the fact the logic board side of the HD SATA and the drives power connectors are the same as the older model. Review Step 22 just to the right of the memory module sockets
You still have to deal with the mounting of the 2.5 HD as it uses a rubber glove Vs screws and you still might have Apples thermal sensor issue to deal with. The IFIXIT teardown shows a HGST drive. You may want to verify if Apple used a standard version or a custom firmware version.
Good Luck! Please let us know what you discover.