I recently purchased a 2012 MBP off ebay. It has a 256gb SSD in the main bay. I want to add a HDD, but am not sure if I should put it in the optical bay (meaning I need to get a caddy and a 9.5mm HDD, right?) or if I should move the SSD to the optical bay and install the HDD in the main bay. The reason I ask is because I'd like to get 1TB of extra storage in there and I don't believe anyone makes 9.5mm HDD's to fit into a caddy for the optical bay. So I would need to get the bigger HDD and only be able to fit it in the main bay. My question really is this: if I move the SSD to the optical bay, can I boot and run the OS from there? Or does the OS only run from the main bay? I've read things about this, but it might only apply to 2011 MBP's and older whereas I have a 2012. In addition, any recommendations for HDD brands or types? If I can't do this, then I will leave the SSD in the main drive and then buy a 750GB HDD + caddy for the optical bay. I appreciate any feedback or help. I am moving on from a 4+ year old Dell laptop that has been giving me BSOD's for the past few months and I'm excited for the MBP to arrive.