I can’t boot my MacBook Pro

I have a 2012 MacBook Pro that wasn’t booting because the hard disk died. I have a clean install of Yosemite on an SSD, which the MacBook starts up from over USB – but not when I install it in the MacBook. I know I didn’t damage anything during the install, because I can take the SSD out and it works over USB. What would stop it booting when the drive is internal? Jonathan Harpen Have you held å while restarting to ensure that you actually have the internal drive selected and the MacBook isn’t trying to start up from a non-existent USB drive? After that, the most likely cause is that the internal drive cable is damaged. They are quite delicate and this might even be why you had trouble in the first place, rather than the drive itself.

If you swap your MacBook’s storage around, it’s easy to damage that delicate ribbon cable.