2013 iMac teardowns reveal SSD slots, soldered-in CPU in the 21.5” model | Ars Technica
Amazon.com: OWC SSD Upgrade Kit for 2011 21.5-inch iMacs, Mercury Electra 250GB 6G SSD, 18" SATA III 6Gbps Data Cable, Installation Tools and iMac Screen Adhesive Tape Set : Electronics