I tried to install windows 7 x64 on a Thinkpad T430 without CSM (the compatibility layer UEFI->BIOS) enabled in BIOS because Windows 7 supports it, but when I boot from install DVD, the installer blocks at loading disk.sys. I tried to disable AHCI or other settings related to discs but nothing happens
If you have Windows 7 already installed on a drive with CSM previously on, then switch it to disabled in BIOS, Windows 7 hangs on startup screen. Is this a bug in UEFI?
Windows 7 requires the CSM (compatibility support module, e.g. legacy mode) to be enabled in BIOS setup. Specifically, Windows 7 video drivers require the CSM. While it is technically true that Windows 7 itself can support a legacy-free (CSM disabled) UEFI installation, not all OEM component drivers were updated for legacy-free.
So, the video driver will not function properly when the CSM is disabled. If you want to use Windows 7 on your system, you have to leave the CSM enabled.
Windows 8 enabled systems (e.g. T430) have a new option in BIOS F1 Setup: on the restart page, there is “OS Optimized Defaults”. Enabled means Win8-only, and Disabled means anything else. So to boot Windows 7, you should choose the OS Optimized Defaults = Disabled, and then press F9 to load setup defaults. Windows 7 can always boot successfully in this scenario.