It automatically/dynamically stores the data you use most frequently in the SSD section, and pushes the data used less often to the slower mechanical SATA drive. If you're looking to get the most space available without losing speed, I recommend ordering the 1TB Fusion Drive. Although the PCIe interface is standardized, Apple is using special firmware on the SSDs using that socket you can't just buy a generic SSD card and expect it to work, and you can't plug a traditional SATA drive into the connector. The downside is, we've lost the ability to easily swap drives in and out. The PCIe bus is much faster than the SATA bus *, so data transfer to/from the SSD is faster. one SATA drive + one PCIe card (1TB 5400 rpm platter drive + 128GB PCIe card = 1TB Fusion Drive).one PCIe card (256/512/1TB SSD flash storage).one SATA drive (500GB/1TB 5400 rpm platter drive).Depending on what you order, you'll either get: Instead, they have one SATA connection for a traditional 2.5' drive and one PCIe slot for a large solid-state flash storage. The current generation minis don't have two internal SATA connections, like earlier generations. Not two traditional 2.5' SATA drives, no.