I recently purchased 2011 21.5 iMac (Mid-2011) and I'm obsessed. : r/mac
Amazon.com: OWC 2.0TB HDD Upgrade Kit for All 2011 iMac Models : Electronics
iMac Hard Drive Upgrade to SSD - Your local Apple Repair Specialists
How can i choose to boot from the second SSD harddisk for a mid 2011 iMac? - Ask Different
Amazon.com: OWC SSD Upgrade Kit for 2011 21.5-inch iMacs, Mercury Electra 1.0TB 6G SSD, 18" SATA III 6Gbps Data Cable, Installation Tools and iMac Screen Adhesive Tape Set : Electronics