I've seen many threads on this and solutions. Unfortunately none help. To cut a (very) long story short. I have two MBP's both 13" 2011 models. MBP(a) failed to recognise HDD. Tried usual diagnostics but could see drive. Thinking HDD failure but just to be sure took drive out of MBP(a) and installed it in MBP(b). Sure enough it booted up just fine. Which was handy as wife could continue to work. Which made me think that there was a fault with MBP(a) itself. So, to confirm i put the HDD from MBP(b) into MBP(a). It also booted just fine. Hmmm. So now we have both MBP running but with the HDDs swapped about. Sooo, once the wife had finished her work I swapped the drives back to their correct machines. Once again MBP(a) failed to boot but MBP(b) worked fine. Grr. Swapped the drives back again to confirm and yep, sure enough when the drives are swapped (i.e. drive from MBP(a) in MBP(b) and visa versa) all is great and both machines work. So, basically MBP(a) will not recognise its HDD but will MBP(b)'s. MBP(b) will recognise and boot of either. Doing my head in as there is no logical explanation as far as I can see. Any thoughts? (oh, that wasn't that short was it). Thanks, Alan.