There is a bug in the 10.10.1 install app, that may cause early reboots on some boards (mostly with HDMI ports). The freshly installed 10.10.1 system is also affected.
Symptoms: you have installed OS X (UEFI or legacy way) and you have installed on the first run all support kexts for your board, as well as your regular DSDTs/SSDTs. On the next boot, the system reboots itself, instead of bringing up the graphics.
There is probably a wrongly set setting that also is being passed to the already installed system, regarding the default audio output and it seems that the wrongly selected output doesn't play well with some corrections like HDMI audio edits, no matter if they are DSDT/SSDT/bootloader HDMI audio injections and that setting causes these early reboots. In order to go outside this boot loop, the audio must be enabled (but without the HDMI audio) and the default audio output must be changed. When you enable the audio, the first thing you'll see in System Preferences / Sound / Output is that the default device is set to Digital Out, instead of Internal Speakers. After you have changed the device to Internal Speakers or Line Out, there will be no more reboots and you can use your HDMI audio corrections again. Note that in order to change the output, the audio must be fully operational.
Since the 10.10.1 installation flash drive is also affected, please avoid using patched DSDTs/SSDTs and bootloader injections, containing HDMI audio edits during the whole installation process. There are several ways to enable the audio without enabling the HDMI audio, all of them described in details by toleda in his Realtek ALC AppleHDA audio guide).
I've seen that issue on two boards (my GB H77N-WiFi and one GB Z77N-WiFi), maybe other board are affected as well.
The issue doesn't occur in the following cases:
- on boards without HDMI ports - for obvious reasons
- on computers where the only way to enable the HDMI audio is a patched framebuffer kext.
- on most laptops - after enabling the audio on most of them, in most cases there is only one outpit and it's the Internal Speakers, even if there are more than one outputs, most of them use HDMI audio only through patched framebuffer kext)
- on all boards/laptops with 10.10, updated manually (App Store or combo/delta update) to 10.10.1 - on such systems the output is already set to Internal Speakers and the update doesn't change that.
Once again: the issue may occur only on freshly installed 10.10.1 system.