Did you possible have more than one hard drive install when you installed Windows 8.1 UEFI?
If so you may have the boot partition on a different Hard drive. This sometimes happens.
So remove all hard drive except the one you are going to install windows 8.1
You already have your rufus set up windows 8 UEFI install correct?
on boot up hit the F8 or ESC key for a boot menu select your
UEFI Thumb Drive start the install as soon as the install window comes select language then hit F10, this will bring up a Command prompt
Use Diskpart to Clean your disk
Use Diskpart to convert to GPT
continue installing windows UEFI
after install reboot Check your BIOS if all is well shut down and connect your other hard drives.
On booting up again hit F8 for the boot menu again and select the disk your windows 8.1 is installed on. you shouldn't have any further problems
Some great step by step Tutorials here
http://www.eightforums.com/tutorials/?filter[2]=Installation%20and%20Setup
Your DATA drives can be either MBR or GPT there really is not much difference unless the drives are over 2 GB then GPT will be needed.
You can convert a data drive from MBR to GPT or from GPT to MBR. But your OS must be installed either one way or the other OS drives can not be converted.
Of course you should always have your data backed up before trying any conversion as something could go wrong during the conversion.
G752VY-DH72 Win 10 Pro
512 GB M.2 Samsung 960 Pro
1 TB Samsung 850 pro 2.5 format
980m GTX 4 GB
32GB DDR 4 Standard RAM
Z97 PRO WiFi I7 4790K
Windows 10 Pro
Z97 -A
Windows 10 Pro