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I had previously dual-booted Ubuntu 13.10 with Windows 8. Then I upgraded it to Windows 8.1. Since then my GRUB has not been appearing. I started Ubuntu from my bootable USB drive and ran boot repair. It initially restored the GRUB. I even upgraded it to 14.04. The GRUB appears as long as I keep using Ubuntu. Even if I once start Windows from the GRUB, the GRUB menu does not appear on the next boot. It directly loads Windows.
Here is the link to my boot info summary http://paste.ubuntu.com/7643346