Marvell controller and Apple Fusion (hybrid) drives
This may be irrelevant due to placement of the harddrive or other factors but it sparks my curosity. The side of the IMAC is only 5mm.
"Marvell is leading the development of 5mm 2.5" hard drives. 5mm hard drives are effectively thinner than a cell phone battery, giving notebook designers even more flexibility in system design, without having to give up storage capacity. "