I want to install Ubuntu 14.04 to dual boot with Win 8.1 installed before. I installed Windows 8.1 in Legacy mode, my laptop is Dell Vostro 2420 which support UEFI but I disable UEFI boot mode before install Windows. Now, when I create Ubuntu 14.04 installation in my usb by Linux Pen Drive, it's UEFI certainly which I can't change. Can I install Ubuntu in Legacy mode and how?
Legacy boot can be turned off after the install is completed - it is only for the usb boot support. UEFI boot is the problem, I'm pretty sure changing that may corrupt the win bootloader. Can't really help you there - re-install?
During the install I believe fast boot and secure boot is what you want to disable in the BIOS. I'm pretty sure I put both UEFI files in the same partition (I could be wrong) - once installed, grub will become your default boot manager.
From there you can go to Windows or Linux - sometimes you have to "fix" the grub - after major Windows updates - I use (http://sourceforge.net/projects/boot-repair-cd/) and do the recommended repair option.