I have iMac late 2009 (EMC 2374) and the optical bay fits well into it. I installed Samsung SSD 850 PRO 256 Gb with a documented speed 520/550 (write/read). But unfortunately it seems because of ODD (optical disk drive) link I've got only 250/270. Anyway I'm pretty happy, because I merged SSD with my hybrid HDD 2 Tb (150/150) and now have 2.25 Tb Fusion drive with about x2 speed. My advise - don't buy expensive SSD with high speed if you want to link it through optical bay - the speed will be cut anyway.
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