Sorry for the late reply, but it's not the size of the drive. I've had a 750GB drive in my mid-2009 MBP for 1.5 years now without issue. '''The behavior you're describing is most likely due to a faulty Hard Drive Cable'''. The first story at the bottom of the '''[product|IF163-013|iFixit page for this cable]''' in question described a fellow who "found this model notorious for the board drive cable going bad." Dozens and dozens of others have experienced almost identical behavior — in [http://www.ifixit.com/Answers/View/32466/Flashing+question+mark+on+startup%2C|this iFixit forum] — in addition to [https://discussions.apple.com/thread/3628282?start=0&tstart=0|this forum] on Apple Discussions, as well as [https://discussions.apple.com/thread/2648100?start=15&tstart=0|this one]. '''RECOMMENDATION: Do the most important test first:''' Remove the drive and see if it functions — either in another unit or as an external drive, with a SATA to USB interface or similar. If it does, then erasing the hard drive and/or reformatting it will not only prove ineffective, but unnecessary. Just replace the hard drive cable (in my case it was '''[product|IF163-013|this one])''' and you should be all set.