Troubleshooting: Apple booting to Prohibitory Sign 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??

Thanks
Prohibitory symbol on a high level means the OS build you're trying to boot is not supported on the machine you're on. This can happen for many reasons, from an actual bad build being installed, to OS corruption, to bad HD. 

Since you said when you swap HDs with another MacBook, both are able to boot, is it possible that they got swapped somehow in the first place? In other words the one giving you the prohibitory symbol actually belongs in the OTHER MacBook? So when you swap them and everything magically works, you're just putting the hard drives in their appropriate Macs.