Reboot loop at Apple logo with Yosemite

[evolutg]

On a fresh install of Yosemite, using by-the-book TonyMac instructions (Unibeast/Multibeast), my hackintosh always REBOOTS when I reach the Grey Apple logo.
But booting from the Unibeast USB drive, Yosemite boots OK!

I have 2 partitions: One with Mavericks and one with Yosemite.
Selecting Mavericks at the Chimera screen boots OK, but selecting Yosemite always reboots with no visible error messages.

I am running out of things to try and fix this.
What is it on the UNIBEAST USB Flash drive that makes my Yosemite boot alright?

Can anyone help? Thanks in advance!

Here is a summary my situation:
- Mavericks boots OK to the desktop, but I never pass the Apple Logo with Yosemite as the system reboots with no error messages, the screen just goes black, then... bam reboot! Between the two partitions, I have everything the same, same BIOS settings, same config files in /Extra folder.
- Yosemite runs OK if I boot from the Unibeast USB flash drive, then select the Yosemite partition at the Chimera screen.
- To boot Yosemite from the hard drive, I need to access the Chimera screen to avoid booting on Mavericks by default, and then select Yosemite partition. That's what leads to the reboot loop.
- When I reach the Grey Apple logo, it only flashes a fraction of a second, then the screen goes black and the system reboots. I do not see any error messages.

I tried verbose mode, but it wasn't informative, it seems to go all the way without any issues, and reboots at the Apple Logo.

I even did a new installation from scratch on a third partition, being careful to select the same settings in Multibeast as I did with Mavericks from my notes (hoping they are accurate), and I still end up in same situation.

One point worth of note, as can be seen in my profile, my processor does not have Internal Graphics. I suspect the issue has to do with switching graphics resolution at the Apple logo, but I am no expert and I am no sure what the system exactly does at that point.

Again, thanks for any help and suggestions!

 

[evolutg]

Never found the solution to this, so I gave up... was it due to my partition scheme? I don't know...
I re-installed Yosemite following the guide on this site step-by-step and this time it worked.

What was different the first time is that I was installing Yosemite on a second partition, because I wanted to keep my first Mavericks partition intact and working fine.

So I was booting from that same drive, but manually selecting the Yosemite partition at the Chimera screen. And then a reboot would occur every time I reached the Apple logo screen.
Now with a single partition and everything else the same (I believe), Yosemite works fine. I installed it on my two systems (see signature).

But I wish I could understand why it didn't work from a second partition, I've done this kind of thing many times before with no particular issues...
Since I use an SSD as the System drive, it so convenient to quickly clone a system to another partition to experiment further without affecting the one that works. I have the user folder for the main user on a different drive (with lots of data), so I am only cloning the system stuff + Applications folder + one admin user (with very little data), and so I only need partitions of 30 to 60 Gb in size.

Could anybody direct me to some good reading to better understand the boot process? How to boot from any partition?