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I have installed a fresh version of Windows 10 while the boot mode was legacy mode in the BIOS. When I change it to UEFI Windows does not boot anymore.

How can I use UEFI mode?

Real Dreams
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1 Answers1

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The simplest way would be to re-run the Windows installation (making sure to repartition your disks along the way) now that UEFI mode is enabled. The Windows 7-10 installers all default to GPT partition schemes and EFI boot loaders when the install cd is booted using (U)EFI.

If you want to fix it without reinstalling, then you're in for some hardship. First, you will need to lay hands on a solid partition editing utility. GParted is passable, but can be quirky... and when editing your partitions quirky is not a good thing. Most of the others, however, cost money. Paragon Hard Disk Manager has worked well for me for years.

Then, backup your data, cause if any of the next bit goes wrong you may not get another chance. You'll need to convert your hard drive to a GPT partition scheme, delete all but your C: partition (specifically, delete the windows system partitions it created during installation), resize you windows partition to be a bit smaller but still be at the very end of the disk (making more room at the beginning), and then switch to the windows install CD and run startup repair. This will let Windows recreate the boot partitions it needs for EFI (which are quite different from what it uses for legacy style booting).

Cliff Armstrong
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    This (what Cliff wrote) is the answer. To amplify: UEFI booting requires an entirely different means of partitioning the disk (UEFI-style partitioning rather than MBR). There is no more master boot record and no volume boot record either. Instead the bootstrap code comes from executable files in an "EFI system partition", which is located by its partition type, which is IDd by a 128-byte GUID. Your existing MBR-formatted disk has none of this, so just telling the firmware to boot with UEFI can't work. https://www.happyassassin.net/2014/01/25/uefi-boot-how-does-that-actually-work-then/ – Jamie Hanrahan Jul 18 '17 at 00:18
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    I would add - if at all possible, get another drive, clone your existing drive onto that one, and THEN attempt to fix the original drive without reinstalling. That way you can always get back to where you are now. – Jamie Hanrahan Jul 18 '17 at 00:22