def get_physical_drives(self):
        """Get all the RAID physical drives on the Server.

        This method returns all the physical drives on the server
        by examining all the controllers.

        :returns: a list of PhysicalDrive objects.
        """
        physical_drives = []
        for controller in self.controllers:
            # First add unassigned physical drives.
            for physical_drive in controller.unassigned_physical_drives:
                physical_drives.append(physical_drive)
            # Now add physical drives part of RAID arrays.
            for array in controller.raid_arrays:
                for physical_drive in array.physical_drives:
                    physical_drives.append(physical_drive)

        return physical_drives