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Guide: Multibooting UEFI on Separate Drives

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Remove everything from the EFI partition on the target drive.
OK... I did delete the old partitions, create new ones, and formatted them. I'd have thought that left them pretty much blank.

Let's back up a sec. I'm concerned I've gotten something wrong here and would like to confirm the general procedure.

Given I have OC/Monterey on the WSD500, I'd like to have Windows on the SDSSD240, and I also have two SATA drives in the mix (for data), would this process be about right?

1. Boot to Monterey on the WSD500
2. Using Disk Utility, partition the SDSSD240 with a 300MB FAT32 partition and the remainder in a mac format (may not matter which type as we'll fix this in the Windows install)
3. Configure BIOS to boot SDSSD240 as the first option
4. Boot to the Windows Install Media (USB)
5. Use diskpart to delete the second partition on SDSSD240 and create a new partition there. Format the new partition NTFS
6. Install Windows with a custom install targeting the SDSSD240
7. Profit.

Q1: Obviously, step 7 is optional and pretty much unlikely, but otherwise does this sound OK?

The last time I tried this diskpart showed my disks to be in this order:
Disk 0, 3726GB (a 4TB SATA drive I use for data storage), GPT
Disk 1, 7452GB (an 8TB SATA drive I use for data storage), GPT
Disk 2, 223GB (the SanDisk SSD I want to install Windows on), GPT
Disk 3, 465GB (the Samsung NVMe I DONT want to touch), GPT
Disk 4, 15GB (I assume this is the USB install disk), GPT
Disk 5, 4657GB (an external USB drive I happen to have attached), GPT

Q2: Do I need to open my case and disconnect the 4TB and 8TB data drives before starting the Windows install? Will it matter if my target disk is not 0? I'd rather not have to do that, but it's certainly something I can do.

EDIT: Hold the phone! I just mounted the EFI from SDSSD240 and it had a whole mess of Apple crap in it! How is that even possible? I used diskpart from the Windows install USB to delete, re-create, and format that partition! Well, now that junk is gone and I'm off to give this another shot!
 
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Rats! That did not help. Got stuck in the exact same place...

I'm reformatting the Primary partition with a low-level NTFS format now and will follow with a low-level format of the EFI partition just for good measure. Maybe that will help? Really not certain...

EDIT: and... that didn't help either. I got a bit further when I used the Windows installer to delete the Primary partition and then aimed at the 223 GB unallocated space. That was really showing some promise as it copied files into it - doing its thing, but ultimately got stuck at the exact same error message, just further along progress bar-wise.

Stumped. What am I doing wrong here, @Going Bald?
 
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Going Bald

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Rats! That did not help. Got stuck in the exact same place...

I'm reformatting the Primary partition with a low-level NTFS format now and will follow with a low-level format of the EFI partition just for good measure. Maybe that will help? Really not certain...

EDIT: and... that didn't help either. I got a bit further when I used the Windows installer to delete the Primary partition and then aimed at the 223 GB unallocated space. That was really showing some promise as it copied files into it - doing its thing, but ultimately got stuck at the exact same error message, just further along progress bar-wise.

Stumped. What am I doing wrong here, @Going Bald?
Procedure is wrong.
1. Boot Mac OS.
2. Format Windows target drive as if you were going to install Mac OS. Shutdown.
3. Boot Windows install media.
4. At the partition to install screen select the Mac OS partition and click delete. This should leave you with the EFI partition and the rest of the disk is free space. Select free space and click continue to install Windows. The only reason for doing it this way is to have the EFI partition as the first partition on the Widows drive instead of the 3rd partition as the Windows installer creates it. With the partition already installed Windows will use it rather than creating a new one. You could also use Diskpart to wipe the drive and create a new partition EFI of at least 200MB if you wanted to do it that way instead. Windows installer will create and format any partitions it needs to complete the instal.

So far as the other drives go, if you can guarantee the boot system of the motherboard sees the Windows target drive before the data storage drives you can leave them connected. I would recommend that you disconnect them and, if possible, disable the M.2 NVMe drive in BIOS. You can re-enable it after Windows is installed. This prevents the Windows installer from installing any files on the other drives. The installer is stupid - it will put the partitions and files it creates on the first drive it sees in the BIOS, regardless of the drive you select to install on. For instance, if you have a motherboard with physical slots S1, S2, S3, S4 and you attach data drives to S1 and S2 and then install Windows on a drive connected to S3, Windows installer will put 3 partitions (Winre tools, Windows reserved and another I forget the name of right now) on the drive connected to S1 instead of S3 because S1 is the first drive the BIOS recognizes and the Windows OS on S3.
 
Joined
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55
Motherboard
Gigabyte Z390 Aorus Pro Wifi
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i9-9900K
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  2. MacBook Pro
  3. Mac Pro
Mobile Phone
  1. iOS
I am soooo stuck. I’ve tried every combo I can think of. I’ve unplugged the internal data drives so SDSSD240 is disk 0, and I’ve tried with an empty EFI folder and with one copied from the Win11 install USB. Depending on how I’ve come at it, I either get the dreaded “Windows could not prepare the computer to boot into the next phase” error right away, or right at the end after installing updates and as soon as it hits Finishing Up.

If I go with an empty EFI folder, then its not possible to set the bios to have the SDSSD240 anywhere in the boot order, and so not first.

Is there a guide on what bios settings I need? Maybe some bios setting I’ve got that works great with macOS is clobbering the Windoze install?
 

Going Bald

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I am soooo stuck. I’ve tried every combo I can think of. I’ve unplugged the internal data drives so SDSSD240 is disk 0, and I’ve tried with an empty EFI folder and with one copied from the Win11 install USB. Depending on how I’ve come at it, I either get the dreaded “Windows could not prepare the computer to boot into the next phase” error right away, or right at the end after installing updates and as soon as it hits Finishing Up.

If I go with an empty EFI folder, then its not possible to set the bios to have the SDSSD240 anywhere in the boot order, and so not first.

Is there a guide on what bios settings I need? Maybe some bios setting I’ve got that works great with macOS is clobbering the Windoze install?
Set the install USB as first in boot order until Windows has completed installing. For best help with installing Win11 try the Windows forums.
 
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