You won't be adding an aftermarket SSD to your new iMac

In its base $1,299 configuration, the 21.5-inch iMac comes with no SSD and no build-to-order storage add-ons; even the $1,499 model only has the option to add a 128GB SSD as part of Fusion Drive, which marries the SSD to the system's 1TB spinning hard disk drive to create a single volume. No SSD-only option exists, nor is there a way to add a standalone SSD.

A scouring of the logic board reveals no spare SATA ports anywhere. The SATA connector used for the iMac's 2.5-inch hard disk drive appears to be the only one present inside the computer.