Troubleshooting: Apple booting to Prohibitory Sign
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.