UEFI Boot fails if legacy mode is disabled completely in settings on ASUS Z87 board

Hi,

on an ASUS Z87 Pro board, the UEFI boot media only work if the CSM mode
in the Settings of the UEFI are set to "both". This means it is using
UEFI boot and CSM mode for legacy booting.

If I disable the legacy CSM and configure "UEFI only", the UEFI loader
still loads but when it comes to load the kernel it just stops and the
system is frozen at that point. Same with 10.1 and 11-current USB
install media.

The system now has FreeBSD 10.1 on one disc installed with UEFI mode and
boots with it as long as the CSM compatibility mode is enabled.

Has someone a similar problem or behaviour?

This leads me to the next question. How can I debug or get more
information what fails at this stage of the boot process? I would like
to provide more detailed information and help to debug to hunt this bug
down.

Regards,

Dominik