My OEM HDD is installed in the "upper" location. I just received my OWC data doubler kit to install a SSD. I contacted OWC and asked if I should leave the HDD in the OEM location or swap the SSD into that upper bay and move the HDD in the lower bay. They said to leave the HDD alone, where it is, and put the SSD in the lower bay with their provided SATA cable. Can you go into disk utility and tell it to boot from the lower bay? I am planning on hooking up the SSD to usb and clone the existing drive over, then mount in the lower bay. I'm just trying to get this process down. I don't want a fusion drive. I want to use the SSD for OS/programs, and use the existing HDD for all my music/photo/office files. TIAFAR.