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How to install with an existing Windows 10 installation to dual boot?

Going Bald

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Hi Going Bald, question for you, related to this post. This is my first post here, and I've read through a lot of the basics sections, but apologies if I've missed this information somewhere else.

I originally built a Windows 10 machine in 2015 or so. (I believe I've got my setup specifics updated in my profile, but I'm basically using an MSI H97I AC motherboard, Haswell CPU, and GTX 750Ti.)

I'm originally a Mac user (I grew up on them), and want to multiboot. I've got 3 SSDs, with Windows 10 already installed on one. The second, 128GB drive, I was planning to use for High Sierra. (I believe that's the limit that I can boot with my hardware and still have graphics support, if I've read things correctly.) The third drive, at 1TB, I intend to use for data shared between both systems. I've formatted that as one large GPT partition with an exFAT file system.

The oldest mac I still have is my old, original, first-gen black 2006 MacBook. I couldn't get High Sierra downloaded from the App Store on there, so I used a friend's 2015 MB Pro to get it, and used UniBeast to create the bootable drive.

The problem I just realized, however, is that, when I checked the BIOS mode on my motherboard, it's Legacy and not UEFI, and I formatted the bootable drive for High Sierra as the latter.

So, my question is, before I attempt the High Sierra install, can I just boot into the BIOS and quickly change the mode to UEFI instead of Legacy, or do I need to make further changes to make the BIOS compatible?

I just want to be sure of this before I start the install. And if you think I may have missed something else, or may be going about things in the wrong way, your wisdom would certainly be appreciated.

Thanks!

Andrew
You can change the BIOS if one of the choices is for Legacy & UEFI.
If your BIOS is now set to Legacy, Windows is installed Legacy mode and cannot be booted from UEFI Clover.
Suggest a change of BIOS to UEFI & Legacy, install Mac OS. When you run MultiBeast post install to install Clover, select the Legacy Mode installation of Clover. Once complete, you will need to boot back to BIOS to reset the default boot drive to the Mac OS SSD. Since you have installed Clover in Legacy mode you will be able to select the Windows icon at the Clover OS pick screen and boot it.
 
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