Window 8 in efi mode and Ubuntu in legacy bios mode

Colin Watson cjwatson at
Tue Oct 22 14:38:53 UTC 2013

On Mon, Oct 21, 2013 at 08:16:32PM -0500, Jim Byrnes wrote:
> I am again trying to dual boot Win 8 and Ubuntu 13.10 on a Toshiba
> laptop, though a different one this time.
> The only way I can get the live DVD to boot to a usable desktop is
> to turn off the efi boot and put nomodeset in grub.
> I've done a lot of googling on dual booting and find many references
> that basically say that both OS's must use the same scheme, ie both
> efi or both bios.  What I can't find is the consequences of not
> doing that. If it means to boot Win 8 I must enable efi and to boot
> to Ubuntu I must disable it, that would be ok, but I doubt it is
> that simple.

In principle this sort of scheme should be possible.  The main gotchas I
can think of would be:

 * Your BIOS needs to support the GPT partitioning scheme.  Contrary to
   popular belief, GPT isn't intrinsically UEFI-specific (though I'd
   argue it's the best thing to have come out of the UEFI committee);
   but some of them do more parsing of the partition table than they
   need to and claim that no operating system is installed.  I've been
   meaning to incorporate a workaround for this into Ubuntu for a while
   but haven't yet got round to it; in the meantime suggests an fdisk-based
   workaround which ought to help.

 * In theory the BIOS calls that read data from disk ought to be able to
   cope with offsets above the 2 terabyte limit (2^32 512-byte sectors).
   In practice they often appear not to, so it's best not to install
   Ubuntu above 2TiB if you plan to boot it in BIOS mode.  This probably
   isn't a problem on a laptop.

 * If you install this way then you won't get a Windows entry in the
   Ubuntu boot loader, or if you do then it won't work.  They should
   still both be bootable, in that Windows' initial boot code will
   reside in the EFI System Partition while Ubuntu's will reside in the
   MBR and the sectors immediately after it, but you might need to keep
   a fairly clear head during partitioning and similar operations to
   make sure you don't break this.

Colin Watson                                       [cjwatson at]

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