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I have new 3TB HDD and motherboard with legacy BIOS. To take advantage of full HDD capacity I have to use GPT partition table. I want to dual-boot Windows 8.1 and Ubuntu 13.10 (64-bit) from this HDD.

From what I find out for now (from AskUbuntu and my own tests) is that Ubuntu can support GPT partition table even without UEFI on motherboard while Windows 8.1 needs UEFI to do that.

  1. Will Ubuntu 13.10 in fact boot from HDD with GPT partition table when only legacy BIOS is available?
  2. If Ubuntu can boot that way, what is stopping Windows from doing so? I know that answer for that question might be outside of the scope of AskUbuntu, but it might give full picture. In other words - how is Ubuntu superior to Windows that it can boot from GPT-partitioned HDD even with only legacy BIOS available?
  3. Is there any way to dual-boot Windows and Ubuntu from 3TB HDD in a way that at least lets Ubuntu use full capacity while leaving Windows with less space?

2 Answers 2

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  1. The answer to your first question is yes, because I am doing it now.
  2. I would guess it's probably a Microsoft design decision to encourage adoption of UEFI, rather than a technical reason. UEFI allows Secure Boot and similar stuff that is more important to MS than to its customers.
  3. I don't know of a way to let windows boot from GPT with a legacy BIOS, but there are a couple of ways you can work around the restriction.

One is to run Windows in a VM under Ubuntu.

The other is to run Windows from a second hard drive, formatted as MBR. Just install Ubuntu on the GPT drive as a single boot system, Windows on the MBR drive as a single boot system, and use the BIOS to pick which one to boot from (on my PC, you just hit F12 during the POST to get a boot device menu). It's a lot easier than mucking around with Microsoft's byzantine BCD scheme. You only need about 30GB for a Win 8.1 pro system partition, and you can buy much bigger drives than that for very cheap. If necessary, you can still have an NTFS partition on your big drive for your Windows data.

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  • Thanks for answer. Actually I can't run Windows under VM, since I need it work as fast as it can since I am using few heavy apps there (mostly Adobe software). Under VM (tested on Virtualbox) it is too slow, so dual-boot is much better option. I run Ubuntu from SDD since it is my main system and I bought that HDD for Windows (and backup Ubuntu installation), so using another HDD is not an option. But I think it might be viable alternative for some users.
    – Rafal
    Dec 14, 2013 at 14:07
  • If I would boot Ubuntu from that 3TB, will Windows see data stored on all partitions of that HDD?
    – Rafal
    Dec 14, 2013 at 14:35
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    As long as the partitions are formatted in FAT or NTFS, Windows can read and write to them. You need Windows 7 or 8 to see partitions above the 2TB limit, but you have that.
    – user225005
    Dec 15, 2013 at 8:23
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I basically agree with user225005 -- the Windows limitation is, in a technical sense, in its boot loader; the BIOS-mode boot loader requires the use of MBR, and Microsoft has decided (for whatever business reason) to not support GPT. In theory, somebody might be able to write a BIOS-mode boot loader for Windows that would support GPT, but I don't know how hard it would be in practice. (There may be things further up the stack that would flake out.)

Using a second drive, as user225005 suggested, is the simplest way to work around the problem. If that's impractical, you might look into using the Developer's UEFI Environment (DUET) or Clover. These are both EFI implementations that can be installed like a boot loader on a BIOS-based computer. DUET is intended for developers and is not easy to install, but the page to which I linked includes pointers that should help. Clover is a Hackintosh (OS X on standard PC) boot loader, which is easy to install from a Hackintosh environment, but there's no documentation about installing or using it without OS X. Both work well on many Intel-based systems, but they don't get along as well with AMD CPUs. FWIW, I'm booting a laptop via DUET, which works reasonably well -- but I wouldn't recommend that the average user set this up unless it's absolutely necessary.

Another option is to use a hybrid MBR. This is an ugly and dangerous hack that enables Linux to see the whole disk via GPT and Windows to see up to three partitions via a modified MBR. Macs rely on hybrid MBRs when dual-booting with Windows, and you could do the same for a Linux/Windows dual-boot. As detailed on the page to which I linked, though, hybrid MBRs are very flaky, and a mistake when using them (such as using a Windows partitioning tool to modify your partitions) can lead to disaster. If it's a choice between the two, I'd recommend you try DUET or Clover first, and then fall back on a hybrid MBR only if you can't get DUET or Clover to work.

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  • Using DUET, Clover or hybrid MBR is too complex and more labor-intensive for my needs. I will use regular MBR (and give up 1TB of disk space) as this is far easier for me at this moment. But those are interesting alternatives which I wasn't aware of.
    – Rafal
    Dec 14, 2013 at 14:12

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