Upgrade Late 2015 21.5" iMac from SATA to SSD/Flash Drive. Back up system with Time Capsule or some other form of backup. I recommend the following equipment and how to video. This flash hard drive: Crucial MX500 1TB 3D NAND SATA 2.5 Inch Internal SSD - CT1000MX500SSD1(Z) (amz0n for $135) With this kit: Replacement LCD Panel Adhesive Tape Strip Sticker + Opening Wheel Tool Kit for iMac 21.5'' 2012 2013 2015 2017 A1418 (amaz0n $10) (There are also other kits that have additional tools if you prefer but aren't necessary) With this How to video: Search on OWC for iMac 21.5-Inch (Late 2012) Hard Drive Install Step-by-step video of how to install a Hard Drive in the Apple iMac 21.5-Inch Late 2012-2015. It's better to format the new SSD with AFMS before installing it but it's not required. I didn't do that. There are USB cartridge devices that allow one to insert and format the new drive externally before installing it.