Why are PCI Express (PCIe) based ExpressCards only recognized by Dell Precision Mobile Workstation M4300 or M6300 systems when the card is present at boot time?

The Dell Precision Mobile Workstations M4300 and M6300 only support hot-plugging of USB-based ExpressCards. PCIe-based cards will only be recognized if they are present at boot time. They cannot be inserted or removed after the system has booted.