Yosemite won't boot in safe mode with Shift held down I'm investigating some slowness with my late '13 rMBP which I upgraded to Yosemite on Sunday. Problem is, I can't boot into safe mode by holding the Shift when restarting or powering up.

I tried everything: Pressing and holding it right after I power up, right after I hear the chime, after restarting, after total shut down. Just doesn't work.

Should I be worried?
Ok, I managed to boot to Safe mode using this: 


  Access the command line by either opening Terminal remotely, or by logging into the computer using SSH.
  
  Use the following Terminal command:
  
  sudo nvram boot-args="-x"
  
  If you want to start in Verbose mode as well, use sudo nvram boot-args="-x -v" instead.
  
  After using Safe Mode, use this Terminal command to return to a normal startup:
  
  sudo nvram boot-args=""


Yosemite is looking more worrisome by the hour though.