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So I have just used gdisk to remove the MBR on my Mac's drive because I got annoyed that I couldn't access my fifth partition - but then learned I could with GPT. Now that I did the "conversion" (its rather just like erasing the MBR and overriding it with a protective MBR) I am facing this one issue that just smells like it's trying to boot with BIOS/MBR although it isn't supposed to:

Couldn't find bootable drive - please insert drive.

Aka. it can't find it's partitio since the MBR is gone.

I don't have my windows install around, so I would like to know if I am able to change Window's boot mechanism from Mac OS? It's Windows 8 that's making the issue.

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Windows will boot from a GPT disk ONLY if its a 64 bit edition and it's running on UEFI.

Most (older)Macs run EFI 1.1, which is not the same as UEFI, which Windows can boot on.

Newer macs, from about 2011 I think, use UEFI (EFI 2.0) and therefore run Windows x64 in UEFI mode just fine.

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My windows is 64bit, and my Mac has EFI and of course my drive is formated with the GUID partition scheme... o.o – Ingwie Phoenix Jun 16 '13 at 10:54
    
Macs actually have UEFI. See this lengthy thread. I myself am actually running Windows 8.1 on an old Macbook5,2 in EFI mode. – kinokijuf Nov 14 '13 at 15:49
    
@kinokijuf Can you point to your specific post? This should actually be impossible... what is your user handle on MacRumours? – Milind R Nov 14 '13 at 18:02
    
I don't know what a macbook 5.2 is, but the very last post in that thread is from 2012, by which time Macs were already UEFI. – Milind R Nov 14 '13 at 18:11
    
MacBook5,2 is a polycarbonate one, afaik from early 2009. Windows 8.1 supports EFI on it, albeit the support is a bit buggy. I did not participate in that discussion, but see this question here of mine. – kinokijuf Nov 14 '13 at 18:57

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