If anyone can help with this issue I will be so grateful; I've spent hours trying to figure this out. Basically, my friend upgraded the internal hard drive in his MacBook Pro from 250GB to 500GB, formatted it using his iMac (Mac OS Journaled), and installed Mavericks on it. The hard drive is a Toshiba MQ01ABD050 500GB. When he installed it into his MacBook Pro, Mavericks doesn't boot, it only shows a blinking folder with a question mark. I thought it might be something with the hard drive cable so I bought a replacement and tested it but it still gives me a blinking folder with a question mark. I then put in the hard drive from another Macbook Pro into my friend's MacBook Pro and it worked. I'm not sure why the new hard drive isn't working? Btw, when I use a SATA cable and plug the new hard drive in via USB it works perfectly. tl;dr Macbook Pro hard drive works externally and OSX runs through USB but not internally. Thanks for reading.