Updated models with AMD graphics options expected in early 2017.
New 27-Inch iMac Disassembled, Aftermarket SSD Install Deemed Difficult
Location of second hard drive bay in new 27" iMac
A number of consumers have wondered about the possibility of aftermarket installation of drives into this bay, should they choose to go with the single-drive option at purchase and perhaps add a second drive later as prices drop and/or capacities increase. To answer that question, Other World Computing has opened up one of the new iMacs and unfortunately found that the task would not be a simple one unless the machine originally shipped with dual drives.
The good news is that the SSD bay available in the new iMacs holds a standard 2.5" form factor drive, which means you can install a high-performance OWC Mercury Extreme SSD for the speed advantage over Apple's SSDs.
The bad news is that, unless your Mid 2010 iMac originally came with an Apple SSD, there's no graceful way to install an OWC one. The appropriate mounting bracket and connector cables are nowhere to be found on the non-SSD models.