GRUB2 menu not appearing on startup

Hi, I'm trying to tu my Ubuntu only laptop (Acer V7) into an Ubuntu/Win 10 dual boot system. I made a Win 10 bootable USB which wasn't recognized by my BIOS as a bootable device. So, I started to make a bootable partition as per this guide when I realized that I wasn't seeing the GRUB 2 menu even after I changed GRUB_HIDDEN_TIMEOUT to 10. What I do see is a blank purple screen for a few seconds before Ubuntu launches. I'm sure I'm probably missing something obvious but I can't figure out what I'm doing wrong. Thanks in advance for your help! My laptop runs on UEFI & GPT.

Boot Info: http://pastebin.com/VTvMfqCT

• sda1 - EFI boot partition
• sda2 - ubuntu /
• sda3 - ubuntu /home
• sda4 - ubuntu swap
• sda5 - empty NTFS for win 10
• sdb1 - win 10 installer (inteal ssd)
• sdc1 - win 10 installer (usb drive)

Is this related to the problem?

Grub2 (v1.99-2.00) is installed in the MBR of /dev/sda and looks at sector 
    25823696 of the same hard drive for core.img, but core.img can not be 
    found at this location.