[Guide] How To Install macOS on top of an existing Windows install on the same drive

Hey, I just wanted to share the steps I took to install macOS on my Laptop, which already has both Windows and Ubuntu installed.

Since it's a Laptop, I only have one drive available, and I believe many are in the same situation where they want to dual boot, but not completely reinstall Windows or other operating systems.

Before you begin, make sure that important data is backed up. I am not a computer expert, just a happy man who messed with his partitions and didn't break it.

It's definitely cleaner to start with macOS installing macOS on an empty drive, but I figured why not give it a try before wiping it anyway?

The main issue with installing macOS on top of an existing installation is the EFI partition, which is where the bootloaders are saved. Windows has one too, but the problem is that it's size is only 100 MB, instead of the 200 MB that macOS absolutely requires.

Here are all the steps I did to successfully change the EFI's size:

1. Create a bootable USB drive with GParted installed. This tool enables you to mess with your partitions. It's installed on a USB drive so that none of the partitions are active when you're editing them.
2. In GParted, shrink your large Windows partition to your new desired size. To do that, right click your large partition and select Resize/Move. I gave it 512000 MiB (500GiB) + 100 MiB (so 512100 in total) on my 1TB drive.
3. Apply changes and wait (Green checkmark in your top row).
4. Resize the Windows partition again. This time, you'll want to set Free Space Preceding to 100 MiB. Your new total size should now be 512000 MiB. GParted will give you a warning that this can break things, and it's absolutely right about that. Apply changes. This operation will take a long time.
5. Move the Microsoft Reserved Partition (MSR). It should be in between the 100MiB EFI partition and your large main partition. You'll want to keep it the same size (I believe it's 16 MiB), so set the Free space preceding to 100MiB, make sure it's the same size, and apply changes.
6. Now we can finally resize the EFI partition to the desired 200 MB. Select Move/Resize one last time, and set the size to 200 MiB. If you can't do so yet, make sure you applied the changes.
7. Create a HFS+ partition in the large unallocated space behind your main Windows partition. If you leave it unallocated, disk utility won't be able to erase it when installing macOS.
8. That should be all! Reboot to test if it works, and if it does, you can proceed to installation!
9. (Optional) You can already modify your EFI in Windows to install OpenCore. Open Terminal as adminsistrator, type

diskpart

list disk (Look for the number of your main boot drive)

select disk 0

list partition (Look for the number of your 200MB EFI partition)

select partition 1

assign letter=a

Now you need Explorer++ in Administator mode to copy over your OC and BOOT folders to the EFI.

Make sure to keep the exisiting files there. I already had a BOOTx64.efi file in my BOOT folder and overwrote it.

10) When installing macOS, open Disk Utility in Recovery, make sure to select "Show All Devices" in the navigation bar and erase your untitled HFS+ Partition as APFS. Now you can follow the normal Dortania Guide.