I have a Lenovo Thinkpad T520 with Nvidia Optimus graphics. Optimus is designed to let you switch between discrete and integrated graphics as needed for performance and battery life.
The BIOS has three options for display: Optimus, Integrated, and Discrete. Integrated and Discrete enable only the Intel or Nvidia chipsets, respectively.
The system boots fine if I selected integrated (or Optimus) graphics in the BIOS, however if I choose Discrete graphics, I'm unable to boot. It appears, weirdly, that when discrete graphics are enabled, Linux is unable to access my hard drive. Here's a screenshot:
Unfortunately, while it appears to dropped to BusyBox, the system is totally unresponsive at that stage and must be rebooted.
The problem persists regardless of whether I've installed Nvidia's drivers, the Open Source Nouveau driver, or both. This is the Nvidia chipset, and the Intel chipset:
00:02.0 VGA compatible controller: Intel Corporation 2nd Generation Core Processor Family Integrated Graphics Controller (rev 09)
01:00.0 VGA compatible controller: NVIDIA Corporation GF119 [Quadro NVS 4200M] (rev a1)