Placing SSD behind optical drive inside iMac 2011 - is a really bad idea! This is the only air channel for a video card’s heatsink. And it’s already partially occupied by the optical drive. There is a lot of place in the central area, where original HDD was placed. And if you are really into upgrading that gorgeous classic, the best thing you can do - throw HDD out. This is also will give you a lot of new free space inside. And also will free up another SATA slot, so you will be able to install a pair of SSD, while keeping optical drive (or 3 SDD w/o optical).