A failing RTC/CMOS battery can cause certain hardware to not appear
On a Dell Studio Slim 540s, I sometimes had the problem of certain hardware sometimes not appearing under certain circumstances 
, such as the USB 3.0 expansion card in the PCIe slot, and even an onboard device being the 1394 controller, despite the latest known BIOS update being installed - at one stage, the computer could not display video from the video card in its PCIe slot, despite OS loading activity.
When I lubricated and reseated the devices on the connectors in question, I took the opportunity to check the RTC/CMOS battery
- it was at 2.7V and warranted replacement.
So far, a new battery fixed this problem, and you may be aware that many modern PCs store PNP (Plug & Play) information in battery-backed extended CMOS RAM (2KB to 4KB, possibly more) - apparently, corrupt PNP information was causing the devices in question not to be recognized under certain circumstances.
, such as the USB 3.0 expansion card in the PCIe slot, and even an onboard device being the 1394 controller, despite the latest known BIOS update being installed - at one stage, the computer could not display video from the video card in its PCIe slot, despite OS loading activity.When I lubricated and reseated the devices on the connectors in question, I took the opportunity to check the RTC/CMOS battery
- it was at 2.7V and warranted replacement.So far, a new battery fixed this problem, and you may be aware that many modern PCs store PNP (Plug & Play) information in battery-backed extended CMOS RAM (2KB to 4KB, possibly more) - apparently, corrupt PNP information was causing the devices in question not to be recognized under certain circumstances.