I have an Intel S3420GP motherboard currently but have seen this happen on other boards as well. When installing an add-in RAID controllers, the controller will show up in the top PCI-ex16 slot and the PCI-ex1 slot on the bottom. However, the add-in cards do not show up in the middle PCI-ex4 slot unless the PCI-ex16 slot is populated.
There is no reason to use the middle PCIex4 slot when the x16 is open, but I can't for the life of me understand why one slot would need populated to enable the other slot. There really isn't a problem here to solve, but I was hoping someone could help me understand this behavior. Thanks for your time.
This is a bit strange,
Can you provide the model # of the RAID card?
The PCIe slots can come off different PCIe bridge devices on the mother board.
Each bridge and down stream device is configured on power-on to establish the path to from the processor to the end card.
During configuration, the each bridge controller in the system has configuration space that contains pointing information to the PCI bus above it and any PCI bus below it.
(here is were the guess work comes in with out knowing more about the card in use)
I suspect that your RAID card contains a PCI, PCIe, or PCIx bridge and the system is not configuring far enough down the PCI bus.
SInce the x16 slot is on the most direct connnection, it is able to configure more bridges below it than say slot 2 or 3 which have to cross addtional Buses to get back to the processor.