Originally Posted by Och
So I'm bumping up this old thread.
I had to change some bios settings, so I was messing with them and then I saw this option that I have long forgotten about. I had it always set to Legacy ROM (which mind you is still a fancy BIOS that has pretty graphics and mouse enabled). So I set it to EFI compatible, restarted the PC - and was presented with blank screen for a while that finally emerged into a blank screen with blinking cursor. No option for me to get back into BIOS setup or anything like that. I had to actually clear CMOS with jumper to get back to the Legacy ROM.
Is this a normal behavior and I'm just doing it wrong, or is there something wrong with my BIOS?
I've never heard of not being able to get into BIOS when switching that mode, that is definitely not normal.
I'm no expert, but do have some experience with troubleshooting similar problems with my x64 based tablet, which has the EFI/BIOS options.
If your OS was installed via the legacy BIOS method then your HDD partition table is using MBR (which has some limitations, so it's not recommended anymore). EFI boot, which uses GPT and a special partiiton just for the EFI, cannot boot via MBR formatted HDD AKAIK.
The same is true in reverse, legacy BIOS cannot boot via GPT formatted drive because it's looking for the MBR, which shouldn't exist.
TLDR: Dont change the EFI/Legacy option on a computer with an already installed OS. Stick with whatever option was used when the OS was installed, regardless of the OS (windows/linux/bsd/etc). If you plan on format and reinstalling, switch it to EFI then before booting from your media.
If you installed via EFI, always keep EFI boot enabled and enable secureboot for added security to the EFI boot partition.
Ultimately, this isn't a setting you just change on the fly.