First I would make sure the firmware is up to date on the hardware on the server. That can be done without an OS. You would launch into the lifecycle conroller (F10) during post and then going into platform update making sure you are using dell ftp site. Make sure your network cable is plugged in when doing so. Next I would make sure I have Dell enterprise drives and that they are updated with the latest firmwar which can be done using a bootable usb key. This to can be done without an OS. Also make sure that you switch back to bios boot mode and that you have a raid configured on your drives.
I'm having the same problem. All drivers are up to date. Can't do an upgrade from 2008 r2 to 2012 r2 or a clean install. It can't get pass the boot manager.