looks like it is a know issue and that method is valid.... If the systems are bound to Active Directory, they are possibly trying to authenticate the user and getting stuck.
Turning off wifi would force them to use cached credentials.
If it continues to happen I will open a ticket with TAM. Tried SMC/PRAM resets and did not fix the issue in our environment.
I still have yet to try Safe Boot on one machine I am currently working on.
I have done the same methods for another Mac Book along with one Mac Book Pro and they all managed to reach the login screen after booting into the recovery partition, turning off Wi-Fi, then restarting the machine.
I don't know if this is the "right" solution for this issue, but other ways to solve this issue would be greatly appreciated as we are planning to upgrade more and more Mac Books (and Pros) to OS Yosemite. I had the same problem with workstations with file vault....
In each case they were bound to AD and were yosemite upgrades.
I found 2 old Canon Bubble Jet USB drivers that was causing out problem. I have had the same issue on MAC mini's without filevault. Wi Fi off, Mc Afee installed, all clean images-new builds, all bound to AD.
It has been sporadic, about twice out of 15 images.
Based on other suggestions online, I've verified that each machine is not attempting to load unsigned kernel extensions.
Other notes: I've also had to halt all additional upgrades until we can determine if this is a Yosemite bug or some other problem, only a problem when upgrading from Mavericks, or some problem with AD-bound machines. What I found so far, this may also apply to non-encrypted machines.
The user could see the mouse cursor, but nothing else.