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I just installed windows with a GPT drive and when I go into BIOS and turn on UEFI mode and restart it just keeps booting straight into BIOS settings. Is there a way to fix this or am I doing something wrong?

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  • @Ramhound Do I need a different installation disk? Jan 8, 2016 at 22:44
  • @Ramhound I don't know where I would find those Jan 8, 2016 at 23:09
  • @Ramhound The .IOS has efi on it Jan 8, 2016 at 23:27
  • @Ramhound Would I just take the boot from the efi file and replace the normal one? Jan 8, 2016 at 23:38
  • replace what specifically? I will again point out how you make a bootable EFI disk is well document. Have you read those tutorials before asking this last question?
    – Ramhound
    Jan 9, 2016 at 0:04

1 Answer 1

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Some observations and comments:

  • Your computer (presumably) has an EFI/UEFI, not a BIOS. I realize that it's common practice to refer to EFIs as "BIOSes," but IMO this leads to confusion, since it encourages thinking of EFIs as BIOSes, which they aren't. EFI was developed as a replacement for BIOS, and it works in a very different way, so thinking of an EFI as a BIOS is likely to lead one to drag in BIOS assumptions that do not apply. Both BIOSes and EFIs have built-in setup utilities that enable you to set various options, and in many cases, EFIs' setup utilities look a lot like those of BIOSes -- but this similarity is only skin deep. One critical difference between a BIOS and an EFI is that the two types of firmware boot in very different ways, using code that's fundamentally incompatible -- sort of like a DOS binary vs. a Linux binary.
  • Complicating the preceding distinction, most modern EFIs include a Compatibility Support Module (CSM), which is a sort of BIOS emulator -- it enables an EFI-based computer to boot BIOS-mode boot loaders. This is analogous to dosemu, which enables Linux to run DOS binaries.
  • Note that an EFI's native boot mode is EFI. In most cases, it's impossible to fully disable EFI-mode booting, although some EFIs do permit this. Rather, in most cases it's possible to enable or disable the CSM, which makes BIOS-mode booting possible (but not certain).
  • When you boot an installation medium on an EFI-based computer, it will boot in either native EFI mode or in CSM-mediated BIOS mode. Which mode will be used can be difficult to predict if the CSM is activated; see my Web page on this subject for details.
  • In most cases, an OS installer will try to install the OS in a way that's compatible with the installer's boot mode -- that is, if you boot the installer in BIOS mode, it will set up the disk with a BIOS-mode boot loader; and if you boot the installer in EFI mode, it will set up the disk with an EFI-mode boot loader. Usually this works fine, but if you're dual-booting and one installer boots in BIOS mode and the other in EFI mode, you're in for an exercise in hair-pulling.
  • Once an OS has been installed, switching boot modes (BIOS to EFI or EFI to BIOS) requires installing a new boot loader, and in some cases making other changes. Simply switching the firmware settings to enable or disable the CSM will not be sufficient. Thus, unless you enjoy jumping through unnecessary hoops, it's best to run your OS installer in the mode in which you intend to boot the OS from the hard disk.
  • Windows ties its partition table type to its boot mode. Specifically, Windows will install to an MBR disk only in BIOS mode and to a GPT disk only in EFI mode.

As a consequence of all the above, your statement that you installed Windows to a GPT disk implies that you did so in EFI mode; however, your subsequent statement that booting failed when you enabled EFI mode suggests that you installed in BIOS mode to an MBR disk. My suspicion is that you were mistaken about the boot mode, and the installer probably converted the disk from GPT to MBR without your knowledge. If I'm right, you can boot normally in BIOS mode by re-enabling the CSM in your firmware. (Using whatever terminology your firmware uses, which might not even mention "CSM" -- the term "legacy" is pretty common, for instance.) Switching to EFI-mode booting can be done, but requires either re-installing in EFI mode or jumping through significant hoops. Your easiest course of action is to just switch the CSM back on and continue booting that way. If you have a compelling reason to boot in EFI mode, please share it.

Also, as a general rule you should not muck about with CSM settings after you do an OS installation. My general advice these days, when doing a fresh install, is to disable the CSM, because it's more likely to cause problems than to solve them. There are exceptions to this rule, though, such as if you want to install an older or EFI-unaware OS (such as Windows XP, DOS, BeOS, or most BSDs), or if you run into a quirk that's specific to a particular boot mode (such as video problems in one boot mode but not another). In your case, if I'm right, you've got a working BIOS-mode OS installation, which is another good reason to enable the CSM.

If you're booting a putatively EFI-capable OS and you have problems getting it to boot with the CSM disabled but not with it enabled, chances are you prepared the boot medium incorrectly. Some tools for preparing bootable USB drives omit the EFI boot loader or otherwise prepare disks that can't be booted on some or all computers in EFI mode. Using another tool, or adjusting options in the tool you used, may work around such problems.

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