I have installed Windows 8.1 Update 1. After that I have installed Ubuntu 14.04 on a btrfs partition. However, after the restart there was no GRUB menu to choose from and the system booted directly to Windows. Could it be the problem with btrfs? I will try to install now on ext4 and let you know if it works.

It's grub/ loader.

And how can I fix it?

reinstall it or use syslinux

I've never had good luck with grub and butter

Hi there

A lot of these problems are due to UEFI systems. If your machine is a UEFI system you need first to DISABLE PROTECTED BOOT otherwise it will only ever Boot Windows.

Now after this you will need to install the UEFI version of your distro / live CD -- you should see this in the BIOS start up menu - there's usually TWO choices for say USB-DISK name and USB DISK (EFI) - Name

Choose the EFI version.

IMO for Dual booting I prefer to install TOTALLY on an external HDD -- Linux runs quite decently from an external USB - then you can run the NORMAL non EFI version and install the bootloader totally on your external drive.

If you really want to dual boot from the Internal HDD then at install bootloader choose the GRUB2-EFI version rather than GRUB2 if its a UEFI system.

Nothing to do with BTRFS -- as far as Windows is concerned it just ignores that partition - same as windows ignores ext4 file systems etc.

Cheers

A lot of web pages suggest to disable secure boot and you may have to. Some vendors may have not made the secure boot normal. Many people can boot to 14 in secure boot and many still claim issues. I'd disable secure boot and see what happens.

Grub has been bypassed it seems or has not been installed.