Installation disk is always uefi

Solution 1:

You've got a BIOS/CSM/legacy-mode installation of Windows, but you booted the Ubuntu installer in EFI/UEFI mode. The two modes can be made to coexist, but doing so is tricky. It's generally better to boot the Ubuntu installation medium in BIOS mode. Broadly speaking, this can be done in either of two ways:

• Learn to control your boot mode -- Most EFIs provide some means to control whether they boot in BIOS mode or in EFI (native) mode. Unfortunately, details vary greatly from one system to another. In many cases, though, when you access the boot manager that enables you to boot from a CD-R or USB flash drive, you'll see two options for the external medium, one of which includes the string "UEFI" and the other of which does not. Select the "UEFI" option to boot in EFI mode and the other one to boot in BIOS mode.
• Prepare a BIOS-only boot medium -- If you prepare the USB flash drive in a way that omits the EFI boot loader, it can't boot in EFI mode. Some tools for preparing bootable USB drives, like Rufus, give you good control over the boot loaders included on the disk. Others don't, although you can usually delete the EFI boot loaders from prepared USB drives. (The key file is EFI/BOOT/bootx64.efi for an x86-64 system.) If you're booting from an optical disc, adjusting the boot loaders on it is much more complex.

Note that these two approaches are intertwined; you can't boot to BIOS mode if the boot medium lacks a BIOS-mode boot loader, no matter how hard you try. Thus, if you have trouble controlling the boot mode, you may need to re-examine how you prepared the Ubuntu installation disk, since that method might have omitted the BIOS boot loader. Also, some computers offer such poor boot managers that you may not be able to control their boot modes except by adjusting the boot medium.

If you need more information on this subject, see this page of mine. Note that it's written mainly for people who have EFI-mode Windows installations and who might be accidentally booting the Linux installer in BIOS mode, which is the opposite of your problem; but the basic information is still relevant.

Another way to solve your problem is to convert your BIOS-mode Windows installation into an EFI-mode installation, as described on this blog post. You could then install Ubuntu in EFI mode. In the long run, this may be a cleaner solution than booting both OSes in BIOS mode, for the reasons described on my CSM page; however, this approach will be much harder and riskier in the short term. Thus, it's probably better to stick with a BIOS-mode Ubuntu installation for now unless you have some other compelling reason to make the switch.