iMac 27 inch Mid 2011 has an additional SSD BTO. You can have one 3.5 inch Hard drive, one optical drive, and one 2.5 inch SSD, however you can have an additional SSD!
Apple did not include a mounter for SSD with non-SSD model, so you cannot add an SSD since it has nowhere to mount. Supports SATA 3.0

Important: Some iMacs may not have mounting points for SSD. Please check if your iMac has mounting points as this image before ordering.
In the case that you have no way of disassembly, then please use this kit.

Internal SSD installation kit for iMac 27 inch Mid 2011.

Package includes:
1. SSD brackets (2 pcs)
2. SSD mount screws (4 pcs)
3. Screws for brackets (3 pcs)
4. SATA data/power cable
5. EMI tape for glass panel (2 pcs)
6. Installation Manual

Installation is not easy.  Expert skills are required. Manual is written for experts, so it is not step by step instructions. If you can disassemble an iMac completely, you can do it.