I've read here and there of a relatively common problem with the 2009-2012, and possibly other, MBPs. What happens is one of the two SATA cables fails and causes problems communicating with the HDD or optical drive. My 2010 2.4 13" MBP had intermittent BSODs in Windows and panics in OS X until it completely stopped booting 2 years ago. A new (actually used) HDD cable fixed the issue completely. The issues happened most when traveling. Now the machine is occasionally not starting (the sleep light just stays on forever), again often associated with movement. I traced the problem to the optical drive SATA cable. Disconnecting and reconnecting it fixes the problem, so the optical SATA cable is likely on the way out as well. What causes these failures? Is it heat? Gravity pulling the connector out? Unibody rubbing off insulation? Instead of buying new cables every few years, I'd like to isolate the issue. I've never had important internal cables fail like this in any other computer I've owned.