SAS3 12Gb/s Technology - SAS3 12Gb/s direct attached storage HDD backplane - 920W Redundant High-efficiency Platinum Level Power Supplies - 24x 2.5" Hot-swap SAS/SATA HDD/SSD bays - Optional rear 2.5" HDD upgrade kit (MCP-220-82609) for mirrored OS - Optimized cooling w/ 3x 80x38mm hot-swap fans; optional 5x60mm middle fan kit: MCP-320-82603-0N - 7x Low-profile expansion slots