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macOS + Win dual boot (at same disk) with OpenCore

Going Bald

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So I'm assuming there is no way to get dual boot working from the same disk if you have a preinstalled Windows Partition while using Clover. I just made the switch to OC 5.9, although not having a simplified way to Dual boot with an existing Windows partition is upsetting. I really don't want to use a USB to boot windows. Any help would be appreciated if there is a simple way.
The problem with preinstalled Win10 is the EFI partition - it is too small - ~90-100Mb. Mac OS wants to see 200Mb or larger to allow the installer to work correctly. This is the same with OC or Clover as the boot loader. OTOH, some Win10 preinstalled do not seem to have an EFI partition even though installed on GUID formatted drive and the Mac OS installer will create one at the end of the drive.

I do know that if you purchase the Windows 10 USB installer (Windows 10 Pro) and use it, it creates only a 90Mb EFI partition as the 2nd partition on the drive instead of the first - the first being WinRe Tools recovery partition when you choose to install Win10 Pro Workstation version. You can also choose several other versions of Win10 - take your pick.
If you are doing a new build, the matter is simple - at the selection screen to choose where to install, open an elevated command window and use diskpart to create a 200Mb or 300Mb EFI partition, exit diskpart and then select the unassigned portion of the drive to install Win10.
 
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I recently set up a dual boot Catalina/Windows 10 system using @Maf3ro’s method but didn’t want to use Bootcamp. I found that after a couple of reboots between the OSs, Windows Boot Manager persisted and the only way to get into macOS was via the bios boot menu or USB stick.

I then tried the bootstrap option (misc>security>BootProtect>Bootstrap). This creates an OpenCore boot entry in the bios ahead of Windows Boot Manager and persists so it always boots into the picker from which macOS, Windows, etc can be selected. I now have a fully functioning dual boot system.

Huge thanks to Maf3ro for his method and explanation and the OpenCore team for their fantastic bootloader. Once you go over to OpenCore, you’ll never go back!
 
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Is it possible after completing this to install something else like Ubuntu? if I want to change the second os after sometime for example.
 
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Am I booting the Windows 10 USB thru OpenCore or thru BIOS?
 

Going Bald

Moderator
Joined
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Messages
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Motherboard
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Classic Mac
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Mobile Phone
  1. Android
Am I booting the Windows 10 USB thru OpenCore or thru BIOS?
You are booting OC, then selecting Win10 icon causes OC to launch the Win10 boot files which then interact with the BIOS same as if you booted Win10 direct.
 
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You are booting OC, then selecting Win10 icon causes OC to launch the Win10 boot files which then interact with the BIOS same as if you booted Win10 direct.
I read that the SSDT from OC/ACPI folder is "applied" to all OS, isn't ?
So it could make a difference from Windows point of view to boot from OC or directly from the BIOS
Correct me if I'm wrong, My knowledge is light.
 

Going Bald

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Joined
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Messages
23,234
Motherboard
GA-X58A-UD7-F7
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  3. Mac mini
Classic Mac
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Mobile Phone
  1. Android
I read that the SSDT from OC/ACPI folder is "applied" to all OS, isn't ?
So it could make a difference from Windows point of view to boot from OC or directly from the BIOS
Correct me if I'm wrong, My knowledge is light.
Win10 is very forgiving of ACPI errors while Mac OS is not. Most SSDTs are corrections of errors or additions to the ACPI to allow Mac OS to work and do not apply to Win10 so have no effect. You might try extracting your DSDT.dsl and compiling it - chances are it will not compile unless you fix errors.
 
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Win10 is very forgiving of ACPI errors while Mac OS is not. Most SSDTs are corrections of errors or additions to the ACPI to allow Mac OS to work and do not apply to Win10 so have no effect. You might try extracting your DSDT.dsl and compiling it - chances are it will not compile unless you fix errors.
Thanks
I didn't know that
I understand what you mean now
 
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