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Since there is no real answer to this really specific case i hope someone here will be able to answer with detailed instructions on how to achieve this.

I have a MacbookPRO (2019) and here is what i did so far :

  • Clean Install of Catalina (10.5.3) (Size is 700GB)
  • Use Bootcamp assistant to Install Windows 10 (Size is 120GB)

Now i would like to install one more Windows 10 on the same disc and being able to choose amongst the THREE operating system at boot time while using the ALT key and being presented with something like this :

enter image description here

The only thing i managed to do so far was to successfully install the second Windows 10 but only being able to choose between them like this (screenshot is from internet) :

enter image description here So my question is (from a fresh install of Catalina on a 1TB internal ssd) how to install two Win10 and be able to choose amognst the 3 operating system directly from the Apple Startup Manager ?

(WITOUTH using rEFInd or other 3rd party boot loader)

Thanks to those who have read me this far, and sorry about my english this isn't my native language..

  • Unless I'm missing something, this is a duplicate question to this one: apple.stackexchange.com/questions/381719/… Please don't post your question multiple times. – Allan Feb 10 at 18:31
  • My first question was actually how to CHANGE the partition related to the boot process with my TWO already installed Windows10 but in this one i'd like to know how to do this from a fresh catalina install in order not to endup with 7 unused partition created by the windows installer.. so i don't think this is the same question but i can delete if this does not respect the rules here. – AppleIx Feb 10 at 18:49
  • Let’s wait to close until we have some answers. Also, this is probably going to get a no answer - you need third party software, but that will help others even if OP doesn’t like it based on the requirements. – bmike Feb 10 at 21:55
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Steps to a Create USB Windows 10 Installer

  1. Use the Boot Camp Assistant application to download the Windows Support Software.
  2. Download the latest Windows 10 iso file.
  3. Use to Disk Utility application to erase a 16 GB or larger USB flash drive. Use the setting shown below.

  4. Use the Finder application to mount the Windows 10 iso file.

  5. Use the Finder application to copy all the files from the mounted Windows 10 iso file to the USB flash drive.

  6. Use the Finder application to copy the downloaded Windows Support Software to the USB flash drive.

Below is the results from a Windows 10 (1909) download and the Window Support Software for a 2018 mac mini.

Procedure to Install a Second Windows 10 Operating System

The first Windows 10 installation should not be encrypted through the use of Bitlocker.

Boot from the USB Windows installation flash drive. Proceed until you reach the same screen as shown in the image below. Click on Custom: Install Windows only (advanced).

The next screen should be similar to the one shown below. At this point, press the shift+F10 key combination to open a Command Prompt window.

The next image shows an opened Command Prompt window.

You need to shrink the Windows partition to make room for the second Windows installation. This can be down by entering the following commands in the Command Prompt window. In this case, commands will shrink the Windows partition (number 3) by 70000 MB (which is 70 GB). You may wish to use a different value.

diskpart
select disk 0
list partition
select partition 3
shrink desired=70000
exit
exit

Below is an example output from entering the above commands.

Microsoft Windows [Version 10.0.18362.418]
(c) 2019 Microsoft Corporation. All rights reserved.

X:\Sources>diskpart

Microsoft DiskPart version 10.0.18362.1

Copyright (C) Microsoft Corporation.
On computer: MINWINPC

DISKPART> select disk 0

Disk 0 is now the selected disk.

DISKPART> list partition

  Partition ###  Type              Size     Offset
  -------------  ----------------  -------  -------
  Partition 1    System             315 MB  1024 KB
  Partition 2    Unknown            722 GB   316 MB
  Partition 3    Primary            277 GB   722 GB

DISKPART> select partition 3

Partition 3 is now the selected partition.

DISKPART> shrink desired=70000

DiskPart successfully shrunk the volume by:   68 GB

DISKPART> exit

Leaving DiskPart...

X:\Sources>exit

After the Command Prompt window closes, you will need to click on the Refresh button. The result should be similar to the image shown below.

Your earlier question indicated your desire to have the two Windows installation not share the same boot files. To accomplish this, each installation will need an unique EFI partition for the boot files. Unfortunately, the Windows installation will not proceed correctly, if the second EFI partition is created at this point. So instead, the space will be reserved for the second EFI partition by creating an unformatted primary partition. My answer posted in the earlier question showns how and when to convert this primary partition to a EFI partition.

Next, highlight the Drive 0 Unallocated Space, then click on the New button. Enter 315 for Size and then click on the Apply button. The result should be similar to the image shown below. Here, a new unformatted primary partition (number 4) was created for future use as an EFI partition.

Again, highlight the Drive 0 Unallocated Space, then click on the New button. Without changing the Size, click on the Apply button. When the popup shown below appears, click on the OK button.

The image below shows a 16 MB Microsoft Reserved partition (number 5) was create along with a new primary partition (number 6).

With the new primary partition (number 6) highlighted, click on the Next button. When the popup shown below appears, click on the OK button. The Windows installation should proceed. This new primary partition will be automatically NTFS formatted before being using as the location for the second Windows installation.

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  • I was just trying again to do it by myself and was able to reinstall both Windows 10 and Catalina after formating my drive but the boot process is still the same.. I'll delete everything right now and try again from a fresh Catalina install with your instructions ! Thanks you so much for taking the time to answer. Will update here if this solve my problem ;-) – AppleIx Feb 11 at 14:40
  • Since i already had the two Windows 10 installed i tried your answer to my first question, but i think i didn't succeeded because i forgot to first disable Bitlocker on the Bootcamp installation before trying to use the bcdboot command to install boot files to the newly created EFI2.. but otherwise everything worked so i'm formating the disk right now and will try again from scratch with the detailed answer you posted above as well as your instructions for creating EFI2 from my previous question. – AppleIx Feb 11 at 15:21
  • I assume you would not want to enable Bitlocker until after implementing both answers. – David Anderson Feb 11 at 23:53

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