2nd HDD/SSD Caddy to replace Macbook Unibody superdrive /optical [OBHD9-SATA-M1] - $10.98 : NewmodeUS, Hard Drive Caddys for Notebooks
Installing MacBook Pro 15" Unibody Mid 2012 Dual Hard Drive - iFixit Repair Guide
Install a 2nd HDD/SSD by replacing the optical drive - Upgrade your old Macbook pro - 9to5Mac
Installing MacBook Pro 15" Unibody Mid 2012 Dual Hard Drive - iFixit Repair Guide
MacBook Pro (Howto): Install A Solid-State-Drive
Installing MacBook Pro 13" Unibody Mid 2012 Dual Hard Drive - iFixit Repair Guide
How to Replace MacBook Pro Optical Drive with SSD: EveryMac.com
Hard Drive Caddy Tray 2nd HDD SSD Kit Compatible with 2.5" 9.5mm SATA HDD SSD 2nd HDD Adapter for Apple MacBook Pro Unibody 13 15 17 SuperDrive DVD Drive : Amazon.co.uk: Computers
Advice from an Apple Tech: How to replace the optical drive in a Unibody MacBook Pro with a second hard drive
Optical Drive to SSD [Macbook Pro mid-2012] | MacRumors Forums