Can I safely use my MacBook Pro HDD late 2011 with a late 2009? I have a GPU issue with my MacBook Pro late 2011 which requires 3 weeks in repair (until the required parts come in). I took out the hard drive and put it in an external casing.

Can I safely use this hard drive on another MacBook Pro (late 2009) and still use my data/applications/OS X?
Yes. According to this document Macs that can run either Mountain Lion or Mavericks are good for Yosemite. The oldest MacBook Pro you can use is therefore Mid/Late 2007. So in any case you should be fine.

OS X Mavericks: System Requirements:


  To install Mavericks, you need one of these Macs:
  
  
  iMac (Mid-2007 or later)
  MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)
  MacBook Pro (13-inch, Mid-2009 or later),
  MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)
  MacBook Air (Late 2008 or later)
  Mac mini (Early 2009 or later)
  Mac Pro (Early 2008 or later)
  Xserve (Early 2009)
  
  
  Your Mac also needs:
  
  
  OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed
  2 GB or more of memory
  8 GB or more of available space