What is UEFI?

[MaxCool]

My computer started installing windows updates (running windows 8.1), and turned off. It never came back on. Said something about the BCD being corrupt.

Well I went into the Bios and noticed "UEFI" was selected as the 1st boot device.

I went and changed it to the HD my OS is on, and now it boots and reboots fine.

What on earth is UEFI, and why won't win8 boot when its selected?

 

[mfenn]

UEFI stands for Unified Extensible Firmware Interface, which is a big long acronym that tells you nothing. In plain terms, UEFI is the replacement for the BIOS. Even when you go into the "BIOS" on your board, you are really going into a UEFI interface.

The UEFI boot option tells the motherboard to attempt to boot in full UEFI mode rather than doing BIOS emulation. If your Windows installation wasn't installed in UEFI mode, it's unlikely to every work in that mode. It doesn't hurt anything to boot in BIOS emulation mode though.