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I had Windows 10 installed, and everything was working great. Updating my Mac OS installation to Mojave caused Bluetooth on my Mac to not work with any other devices except my iPhone.

I was hoping 10.15.4 would fix my Mac. I updated, and now I cannot boot into Windows 10. It either restarts, or it goes to a screen giving me the choice of Advanced Repair options or trying to continue to boot (which results in restarting).

Mac OS of course, still seems to work fine. How can I boot into Windows 10 again ?

Kaizer Sozay
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1 Answers1

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You can try recreating the Windows boot files. This answer assumes you are UEFI booting Windows 10.

To rebuild the boot files you need to boot to Windows Recover Environment (WRE) of from a Windows installation flash drive. If you can boot to Windows and have a properly working WRE, then restart Windows while holding down the shift key. You will need to navigate to where you can open a Command Prompt window. Otherwise, to create a Windows 10 installation flash drive and then open a Command Prompt window, follow the steps below.

Create a Windows 10 Flash Drive Installer

  1. Download the latest Windows 10 ISO.
  2. ExFAT format a flash drive using the MBR partition scheme.
  3. Mount and copy all the files from the ISO to the flash drive.
  4. Use the Boot Camp Assistant to download the Windows Support Software.
  5. Copy the Windows Support Software to the flash drive.
  6. Boot from the flash drive.
  7. Proceed as if you are installing Windows until asked for a product key.
  8. Press the shift+F10 key combination to open a Command Prompt window.

Rebuild Boot Files

Once you have a Command Prompt window open, enter the following commands.

Mount the EFI volume and assign to drive S:.

mountvol s: /s

Remove any previous backup. If no previous backup exists, then expect error messages to appear.

rd /s /q s:\efi\Microsoft.old
rd /s /q s:\efi\boot.old

Backup existing files by renaming folders.

ren s:\efi\Microsoft Microsoft.old
ren s:\efi\Boot Boot.old

Determine the existing drive letters. Determine by inspection the drive letter of the main Windows volume. Usually, this is drive C:. If you are using a different drive letter, then make the appropriate substitutions in the subsequent commands.

echo list vol | diskpart

Create new boot files.

bcdboot c:\windows /s s: /f UEFI

Delete the Boot Configuration Data file (BCD) created in the previous step.

del s:\efi\microsoft\boot\bcd

Rebuild the BCD. Answer yes to any questions.

bootrec /rebuildbcd

Close the window.

exit

References

agarza
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David Anderson
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  • Thanks for your answer. I can't verify if its correct or not because Windows seems to have arbitrarily decided to boot now. But I'm marking it as correct so other people can try it if they have the same problem. – Kaizer Sozay Apr 11 '20 at 03:48
  • Actually the problem has returned. Arbitrarily boots into Windows either succeed or fail. Windows is unable to automatically repair or diagnose the problem when I choose advanced repair options. I think trying this is risky, because the problem seems to be with something Mac OS is doing (not every time) – Kaizer Sozay Apr 11 '20 at 11:27
  • To repair windows you usually have to do this offline. This means you need to boot to a small version of windows that is running from a RAM drive. You have yet to do this which may be why you have failed to repair Windows. – David Anderson Apr 11 '20 at 13:07
  • I’m narrowing down the cause - it’s not random - I’m thinking launching specific software on OS X causes the problem. – Kaizer Sozay Apr 11 '20 at 13:09
  • It seems to be a problem with Mac OS - 10.15.4 has many bugs which are not only bricking macs but causing problems like mine. – Kaizer Sozay Apr 12 '20 at 17:54
  • macOS has nothing to do with Windows. Microsoft Windows installs on a Mac the same as any ordinary PC. The instruction given in my answer would apply regardless if you own a Mac or some other brand computer. – David Anderson Apr 12 '20 at 21:53
  • although Windows and Mac OS are on different volumes / partitions, there are some thing related to booting that are shared. Mac OS seems to be messing with some setting to arbitrarily you can’t boot into Windows.
    • – Kaizer Sozay Apr 13 '20 at 02:38
  • Seems to have something to do with the NVRAM. Resetting the NVRAM causes instability. – Kaizer Sozay Apr 19 '20 at 15:47