<< Solved >> Windows 10 (GPT/UEFI installed) wont boot with CSM disabled

[Blableuh]

Hey,
I always had this problem with this build (got it since about 6 months ago) and I still could not find any solution.
I have macOS Mojave and Windows 10 installed on separate M.2 SSDs. I would like to disable CSM in BIOS in order to get rid of the ugly resolution I get during the computer startup/POST/Clover menu/OS loading, and maybe later try to switch from Clover to OpenCore (which need CSM disabled), and also because I don't see why CSM should stay enabled for my configuration.

When I disable CSM everything runs fine except that Windows wont boot: stays on a black screen, or sometimes I get the ASRock logo back with "Prepairing automatic repair" message but with no further evolution. I tried to boot Windows from the Clover entry and from the UEFI boot manager but the issue is the same.

I've read everywhere that if you want to fully disable CSM, Windows need to be installed in UEFI mode on a disk with a GPT partition table, which is the case for me (I triple checked in the disk properties for GPT and in msinfo32 for UEFI mode. I also checked for any extra sneaky MBR partition but I could not find one). When I installed Windows I had nothing but the installation USB stick and the Windows m2 SSD pluged into the same slot as is now. I then pluged the macOS SSD and installed macOS following this guide.

The setup:
- motherboard: ASRock Z390 Phantom Gaming ITX
- CPU: i7 9700 (non-K version)
- GPU: RX590 Sapphire Nitro+, monitor connected through HDMI (would prefer DP but that's another story/issue)
- RAM: 2*8GB 3200MHz Corsair DDR4
- SSD 1 (front m2 slot): NVMe Corsair MP510 with Clover 5100 partition and macOS 10.14.6 install
- SSD 2 (back m2 slot): SATA Crucial MX500 with Windows 10 installation

I don't know where to dig anymore, I would be very grateful for any clue!

EDIT: Disabling Above4G decoding in BIOS ade everything work as expected: I can now properly boot macOS and Windows with CSM disabled and iGPU and dGPU support in both OS. Got the clue by reading this post.

 

[Going Bald]

Boot Win10, run disk management tool, take a screenshot of the window and post.

 

[Blableuh]

Disk 0 (CT2000MX500SSD1): 2.5" storage disk with two 1Tb partitions (one APFS and one NTFS). I just noticed the 200Mb EFI partition on this drive, idk why it's here, but I just checked it and it's empty. And my problem was the same before I install this disk.

Disk 1 (CT500MX500SSD4): The M.2 SSD in the back of the mobo. Windows disk with recovery partition and Windows boot manager

Disk 2 (Force MP510) : M.2 SSD in the front slot of the mobo. macOS disk with EFI partition containing Clover.

 

[Going Bald]

Check for presence of Windows boot files in the EFI partition of disk 1.
Make sure you have not hidden any Windows partitions in your config.plist.

When you boot Clover, at the pick screen is there an icon labeled boot Windows EFI from EFI?

 

[Blableuh]

Everything seem clean:

• No Windows boot files in the EFI partition of disk 1 (the 3 folders inside of EFI are APPLE, BOOT and CLOVER)
• I deleted all hidden volumes in config.plist and in Clover I still have only one Windows entry which is the same as before: "Boot Windows EFI from EFI".

However I noticed that I can properly boot Windows with CSM disabled if I
- disable IGPU Multi-monitor (normally enabled for macOS to make it use the iGPU as expected in iMac19,1)
- Set "Share Memory" to Auto (normally set to 256M)

So a first guess is that the issue may come from the dGPU and/or iGPU?
I also think I could bypass this problem by not using the iGPU at all and switching to iMacPro1,1 but I don't really like this idea as I want to keep my dGPU as cool as possible in macOS so using iGPU for some tasks seems like a must-have to me.

 

[Going Bald]

Everything seem clean:

• No Windows boot files in the EFI partition of disk 1 (the 3 folders inside of EFI are APPLE, BOOT and CLOVER)
• I deleted all hidden volumes in config.plist and in Clover I still have only one Windows entry which is the same as before: "Boot Windows EFI from EFI".

However I noticed that I can properly boot Windows with CSM disabled if I
- disable IGPU Multi-monitor (normally enabled for macOS to make it use the iGPU as expected in iMac19,1)
- Set "Share Memory" to Auto (normally set to 256M)

So a first guess is that the issue may come from the dGPU and/or iGPU?
I also think I could bypass this problem by not using the iGPU at all and switching to iMacPro1,1 but I don't really like this idea as I want to keep my dGPU as cool as possible in macOS so using iGPU for some tasks seems like a must-have to me.

try setting the iGPU shared memory to 64M or some other multiple of 16 instead of 256M

 

[Blableuh]

I disabled CSM, kept IGPU multi-monitor enabled, and tried every available value in my UEFI for "Share memory" (32, 64, 128, 256, 512, 1024), but same result: black screen, and one time over two the message "prepairing automatic repair" appears but nothing happens (I waited "only" for 10 minutes).

 

[Blableuh]

I would install windows 10 first, make sure that you choose to install windows in uefi mode, i guess you installed windows in bios mode, thats why you need csm...After a complete windows 10 installation then install clover and osx...there is a old dual boot guide on the forum which helos understanding that

This is exactly what I did. Twice.
As said in my first post, beside Windows not booting with CSM disabled, there is no sign that Windows is installed in BIOS mode. The Windows disk has a GPT partition table (not MBR as it should be for legacy BIOS mode), and running msinfo32 shows "UEFI" in "BIOS mode".

 

[Blableuh]

Have you tried run bios first with csm disabled before you install win 10?

This is the question I'm asking to myself since I posted this thread. I installed my build a few months ago and I don't remember if I succeeded to install Windows 10 with CSM disabled. For sure I remember I had hard time to make the USB media installer to boot properly with CSM disabled, but I don't remember if I figured it out eventually or not. I also remember I spotted the CSM issue decribed above quite quickly, and that's why I tried to re-install Windows 10 a couple of times (with macOS disk unplugged) but I got the same result every time.
I know some people (seen in this thread)with a similar build but they did not encountered that issue so now I'm totally clueless, Idon't know where to dig anymore.

EDIT: I was re-reading your question and I'm actually not sure if I understand it correctly. What I understood: "Did you try to install Win 10 with CSM disabled in UEFI/BIOS ?"

 

[Blableuh]

I used this guide back then https://www.tonymacx86.com/threads/guide-multibooting-uefi-on-separate-drives.198869/

I am sure that I did the win 10 installation media with the windows 10 media creation tool. However, I still have the USB stick I used for that purpose, I think I did not used it since then, but I just checked its properties and it says its partition type is master boot record, instead of GUID/GPT I guess.

This said, I rechecked my windows install and as you can see it says UEFI: