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I have my Windows install on a USB, the installer says I can't install on my HDD (GPT) harddisk because I am booting from USB? How can I fix this?

What kind of logic is this? If I boot from DVD I can install on GPT else I can't?

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Are you trying to install windows, or a program onto the USB? – soandos Dec 25 '11 at 9:29

Microsoft erroneously conflates has an EFI partitioned hard disc with has EFI firmware. This is, of course, clearly wrong. It's quite possible — and indeed is becoming ever more desirable these days — to have an EFI partitioned disc on a machine that has old non-EFI firmware.

One of the several consequences of Microsoft's error is that the Windows NT 6.1 installer has to be invoked from an install medium that has in turn been bootstrapped from new EFI firmware, in order for it to accept the idea of installing Windows NT 6.1 to a disc partitioned with the new EFI partitioning scheme. Unfortunately, if the Windows NT install disc is bootstrapped in the old PC98 way, as you've probably done with your USB disc, the installer will think that there's old PC98 firmware, and so declare that it cannot be installed to EFI partitioned hard discs.

As the Microsoft documentation explains, the installation CD-ROM is in fact dual-boot. A machine with old PC98 firmware will bootstrap one operating system image and installation program; and a machine with new EFI firmware will bootstrap another. As Rod Smith explains, one therefore has to manually construct a Windows NT 6.1 install disc that bootstraps in the new EFI way. The Windows NT 6.1 installer will then allow installation to an EFI partitioned hard disc.

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Observe the following paragraph from this MSDN article:

Note: Starting with Windows Vista, you can install a Windows x64-based operating system on a GPT disk only if the computer has UEFI boot firmware installed. However, installing a Windows x64-based operating system on a GPT disk is not supported on Windows XP. Attempting to do so yields an error.

Maybe you used traditional BIOS to boot the Windows installer on your USB disk which prevented it from detecting whether UEFI is available. You'll need to make sure UEFI is enabled in your BIOS and the correct .efi file is present on your USB key.

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I used multisystem to build the usb. How do I check if the correct .efi file is in? I am using ASUS H67M PRO which supports EFI I think, not sure if its disabled somehow, how do I know? – Jiew Meng Dec 25 '11 at 11:15
Based on some basic inspection, I think multisystem uses traditional BIOS rather than UEFI. You'll have to make sure the "efi" directory is copied from the original x64 Windows DVD to the root directory of the USB drive. Then you'll have to enable UEFI in BIOS settings. You may get better results if you update your BIOS (and UEFI firmware) first. Note that the support for UEFI is growing but still limited. You may make your life easier if you just change the partition table type to MBR. – Dec 25 '11 at 12:21
I am leaving the change to MBR as a last resort as I need to backup all my files to change the partition table. The files appear to be there, also I notice there are 2 options for the USB, 1 the classic, the other prepended with "UEFI: ..." when I try the later, it goes back to the BIOS. I also updated my BIOS recently – Jiew Meng Dec 25 '11 at 13:33
Do I need to do something like – Jiew Meng Dec 25 '11 at 13:33
I'm not exactly sure about UEFI boot from USB as I've never tried it. Maybe if you remove the BIOS-based boot records on your USB drive, you'll be able to boot UEFI. (You'll probably need to delete the partition map on your USB.) Also you can always go back from GPT to MBR without losing any data because the former reserves enough space at the beginning of the disk. Most modern GPT-aware partition tools (including Disk Manager in Windows) allow you to do this. – Dec 25 '11 at 13:41

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