The FlashArray//XR4 introduces a new chassis layout and PCIe slot numbering scheme compared to previous generations. For expansion connectivity, specific slots are designated for the backend SAS or Ethernet/RoCE fabric cards that connect to DirectFlash Shelves.
According to the //XR4 hardware guide and expansion best practices, PCIe slot 3 is the designated location for the additional expansion interface card required when adding a second DirectFlash Shelf (or for specific shelf configurations). Slots 0 and 1 are often reserved for host I/O or other functions, and slot numbering/priority is strict to ensure the BIOS and Purity software correctly recognize the card function.
Installing the card in the wrong slot (like slot 0 or 4, if those are reserved for other uses or primary I/O) could result in the shelf not being detected, the array failing to boot, or suboptimal performance due to PCIe lane allocation. Therefore, the engineer must install the DFS expansion card into PCIe slot 3 and cable the second shelf to the ports on that specific card to ensure a supported and functional configuration.