Retina iMac internal storage configuration and possible upgrades?

Yes, the pure SSD models come with a pcie SSD and although you may think that they leave an HDD bay empty apple has gotten into the habit of not cabling up components that aren't needed on the base model (mac mini). So if you want to get the the 5K imac upgraded DIY, the best option is to get the base option and then possibly upgrade the HDD to a 6TB etc. and the pcie SSD to a 1TB apple of ebay.

 

The HDD bay is left empty and it actually helps cool the iMac's temps down by allowing more air to circulate. An HDD can be added later but it will add its own heat and make the system run hotter.

I've always argued in favor of an SSD only system with external HDD storage. That way if the HDD ever fails it can be easily swapped for a new one. It can also be swapped for one with a larger capacity at any point.

 

SSD drives fail too, quite often. I have quite a collection of Samsung, Crucial and Kingston SSD drives from customers waiting for RMA's

 

Note, BTW, that the options for the riMac's a somewhat different than the regular iMacs in that you get either 1TB Fusion or 256 SSD only; no extra charge for those. Although of course the price is factored in.