The TrueNAS Mini R has a PCIe 3.0 x8 expansion slot

The TrueNAS Mini R gives you a PCIe 3.0 x8 slot for expansion — throw in a network card, a storage controller, or a more capable HBA. It’s a proper small-form-factor NAS that doesn’t lock you into the stock config. That slot makes a real difference if your storage needs grow beyond the built-in ports.