Hello and welcome to the forum.
This is going to sound stupid, but since it just happened to me the other day when installing Windows 8 RTM from an attached USB HDD.
After the install completes and before the computer boots to the first run setup / final stage of the install process...... Do whatever you need to do to get back into the BIOS and check your default disk boot order.
Sometimes and I assume on some boards with some BIOS utilities, when you evoke the Boot Option menu during POST by tapping the F12, F11 or whatever key more than one time it will actually reset the default boot order in your BIOS to the Boot Device you have chosen from the Boot Menu, don't ask me why but it has happened to me more than once.
Hope this helps
Regards
Randy