Clean install broke my dualboot

[PBarrette]

I had my system dualbooting nicely between Windows 8.1 and Windows 10 Preview. I decided to do a clean install of Windows 10, and now I can only boot to Windows 10. I've disconnected all my drives, leaving only the two Windows drives. If I disconnect the Windows 10 drive and leave only Windows 8.1, it will not boot at all, and Startup Repair via a USB doesn't work. If both are connected only Windows 10 boots, and only if that drive is set as the primary boot drive.

A few notes:

• I've tried EasyBCD to fix the boot, and it didn't help (actually, it made it worse)
• I've also tried many configurations in the BIOS, with no success
• The Windows 8.1 drive was MBR, I converted it to GPT after this problem started
• I booted to a Windows 8.1 USB and Repair Windows couldn't work
• Before I had the nice Windows dualboot screen, now I get the old black and white boot screen with no mouse support
• I did reformat my Windows 10 drive (SSD) before installing Windows 10

Below is an image of what the drives look like.

My motherboard is an Asus Maximus VI Gene.​