I'm running a late 2011 MBP 13". 500 GB HDD, 16 GB RAM. Otherwise stock. Has been super reliable. Mostly web browsing, email, Word/Excel, some minor video editing and photo editing. Not a graphics design/web guy. So it's not getting heavy use. I noticed this weird black block in the upper left hand corner of my screen. Restarting, putting it to sleep does not make it go away. Should I be worried? HDD failure? Integrated graphics dying? Motherboard gasping for life?