You will need to choose which mode you want to install Server 2012 prior to the installation.  You will need to reinstall the software in UEFI mode.  The partitioning is different in each one.     Regards, 
I just installed windows 2012 STD edition and created a raid 1 using h700 perc card. I went back now to change boot settings from BIOS to UEFI and it can't find the harddisk c, I tried to add boot option in UEFI bootmanager but I don't see the hard disk c or perc listed. why?
UEFI says no bootable device