Grub won't start Windows 10 and there is a "Windows failed to start" error in Windows Boot Manager

Question

I have had dual boot for a while now, having Windows 10 and Ubuntu 15.04. It was working perfectly. When I turned on the PC Grub would show up with Ubuntu and Windows 10 as options, both worked perfectly.

After messing around with Ubuntu l decided to reinstall it, so l downloaded Ubuntu 16.04, made a bootable USB and installed it over Ubuntu 15.04.

The problem is, now that I have Ubuntu 16.04 Grub won't load Windows 10, instead it displays an error as shown below.

Windows Boot Manager  

Windows failed to start. A recent hardware or software change might be the cause.  
To fix the problem:

  1. Insert your Windows installation disc and restart your computer. 
  2. Choose your language settings, and then click "Next."
  3. Click "repair your computer."

If you don't have this disc, contact your system administrator or computer
manufacturer for assistance. 

  File: \Boot\BCD  

  Status: 0xc000000a  

  Info: The Boot Configuration Data for your PC is missing or contains errors.

The curious thing is, I CAN boot Windows if I change from Legacy to UEFI or by pressing F12 (boot manager) and choosing Windows from the options.

It used to work fine and I don't know how I messed up.

Answer 1

If you reinstall Ubuntu 16.04 as UEFI then it should sort it out.

I was used to legacy boot but finally gave UEFI a try with a newish laptop. It works fine but what I didn't realise initially, when I was trying to install Ubuntu from a USB stick, is that I had to press f12 on booting up (to offer me the 'one-time boot menu') and then select the stick under the UEFI options.

If I didn't do that then it just read the stick as a legacy boot stick and, if I proceeded to install it in the usual manner, then Ubuntu and the grub bootloader would be installed in 'legacy' mode.

Since Ubuntu and grub are then following the Legacy Boot rules, they don't have 'permission' to boot up a UEFI Windows partition and so you get the error.

If Ubuntu is installed in UEFI mode, then grub will be able to boot a UEFI Windows partition without any problems.

Your grub boot menu (apart from the 'extra' Ubuntu 14.04 that I've installed) will probably then look something like this: