Hello, Upon starting up my MacBook Pro (mid 2010), the loading screen appears and after a time I am presented with an error message and then it restarts (this process would continue if I left it). It's worth noting two things that happened out of the hundreds of times I must have booted up: 1.) I did manage to get it to boot into OS X once, but it was very slow and unresponsive - for example, Safari did not load any pages, and I couldn't move any files due to an error "code -50". 2.) It's also worth noting that on a handful of occasions it appeared to get past the boot-up loading screen, and I was presented with the message "Unapproved Caller / SecurityAgent may only be invoked by Apple Software" (this message is never sated so I ultimately have to restart). I have tried booting into safe mode but this seems not to be successful. Attached a picture of the boot-up error message I initially described. I'd welcome any suggestions or comments - thank you.