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If you look at your configure options on the new high end iMac, you see that you have options for HDDs, SSDs and combinations there of.  That means there are two places for hard drives in the new models, one likely being a smaller 2.5-inch SSD bay. 

That also means that if you buy a new iMac with just one drive, you can install another aftermarket drive (SSD/HDD opposite) inside.

 Thanks JoeR!