Se você estiver usando dual-boot com o Windows 8, devo assumir que você precisa do UEFI ativado. Então, para resolver seu problema, você deve usar o Ubuntu 12.10 64-bit ou posterior, e seguir o Ubuntu wiki sobre UEFI :
To install Ubuntu in EFI mode:
- Use a 64bit disk of Ubuntu (32bit installer does not detect EFI)
- Use the last version of Ubuntu. Support for UEFI appeared in 11.10, but has become more reliable in next versions. Support for UEFI SecureBoot appeared in 12.10 and 12.04.2.
- Set up your firmware (BIOS) to boot the disk in UEFI mode.
- Then:
- nothing special is required if you use the automatic installer of Ubuntu ("Install Ubuntu alongside others" or "Erase the disk and install Ubuntu"). Important: if you have a pre-installed Windows and you want to keep it, do not choose "Erase the disk and install Ubuntu".
- if you use the manual partitioning ("Something else"), the difference is that you will have to create and use an EFI partition.