Physical slot:
PCIe 3.0 x 16 1slot (PCIE6/4: single at x16 (PCIE6); dual at x8 (PCIE6) / x8(PCIE4)
PCIe 3.0 x 8 1slot (PCIE4)
PCIe 3.0 x 4 1slot (PCIE7: shared with M.2)
3.0 x 1 1slot (PCIE5)

Due the PCIe 3.0 x 8 slot share from x 16, thus for best expandability the PCIe x 8 slot for HBA card (link HDDs) used, and able to support an additional PCIe x 8 card only.

TYPE1: (current usage)
PCIe 3.0 x 16 1slot (PCIE6/4: single at x16 (PCIE6); dual at x8 (PCIE6) / x8(PCIE4)
PCIe 3.0 x 8 1slot (PCIE4) (HBA card occupied)
PCIe 3.0 x 4 1slot (PCIE7: shared with M.2)
PCIe 3.0 x 1 1slot (PCIE5)


TYPE2: (HBA card invaild unable to detecet any HDDs)
PCIe 3.0 x 16 1slot (PCIE6/4: single at x16 (PCIE6); dual at x8 (PCIE6) / x8(PCIE4)
PCIe 3.0 x 8 1slot (PCIE4)
PCIe 3.0 x 4 1slot (PCIE7: shared with M.2)
PCIe 3.0 x 1 1slot (PCIE5)