So my macbook pro (2012 Intel i5 6gb ram), has been starting up to the prohibitory sign. This sign will kinda sway back and forth between the apple symbol and the prohibitory sign. Every once in a while it will stick at the Apple Logo and turn on.
I have reinstalled the OS, reset the NVRAM, and pretty much everything else I could think to check, including hardware diagnostic checks.
I work as an IT and happened to have another Macbook pro of the same year and model laying around. When I switch the hard drives in the two computers both computers, both of them are able to boot up normally every time. (This confuses me).
Any idea on the problem or other things to try??