I know this is a strange question but I needed to ask it. Can a PCI Express x1 card work in an x4 slot when that slot is in x1 mode? I know PCI Express is modular in a way and also the first notch will allow the card to slide in but I was wondering if the motherboard would recognize the card. Like most of you, I have a dual slot GFX card that covers the x1 slot, but my x4 slot was spared. Unless I had a fiber optic 4Gbps network or a huge raid setup, I have no use for an x4 slot but there are several x1 cards I would love to own. Heres one: http://www.newegg.com/Product/Product.aspx?Item=N82E16815129006
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...Thanks to Dell, I just found my answer: YES. Thats quite interesting. Dell has some good tech write-ups on their site!
Figure 7. PCI Express System Board Connector Size for Standard and Low-Profile Cards
Table 3 shows the interoperability requirements of standard and low-profile PCI Express cards. A x1 card can be used in all four system board slots: x1, x4, x8, and x16. When a x1 card is inserted into a higher-bandwidth slot, the link layer negotiates the link down to the x1 data transfer rate.
Yes, that's per the PCIe spec. All slots are required to support cards with the same number of lanes, and x1 cards. Technically, there's no requirement for (say) an x16 slot to support an x4 card, but because of the way the devices negotiate lanes it always works (AFAIK). (There's also a funny wrinkle in the inital spec for supporting x8 cards in slots that only have 4 data lanes but are physically x8 slots, because of early chipset implementations that couldn't support a lot of lanes).