What do you mean "Won't boot" ? What error do you get ?
If you installed it using SATA/IDE mode, and now it is set to AHCI, then it will BSOD, you will need to make sure the SATA Controller is set the same as it was when you installed windows
MOBO - Crosshair V Formula
CPU - AMD FX-8350 @ 4.8GHz
COOLER - Corsair H100i / MX-4
RAM - 12GB G.Skill RIPJAWS X @ 2133MHz
GPU - XFX R9 295X2 8GB
HDD - 2TB Seagate
PSU - Corsair TX950W
CASE - Coolermaster HAF 922
OS - Windows 8.1 Pro 64bit