Macbook Pro not detecting bootable USB with Yosemite
You might try resetting the pram on your mac.
I understand that startup disk settings are stored there.

If the disk is bootable on other Macs, it stands to reason that it should boot on this one. 

You can reset the pram by doing the following:


Shut down the Mac. 
Locate the following four keys:  option,
command, p and r. You'll need them in the next step. 
Power on the Mac, then immediately hold option, command, p and r.
You need to do this before the grey screen appears.
Hold down the keys until you hear the startup sound twice, then release them.
Plug in the bootable USB stick, restart the Mac, and hold option.


I can't guarantee that it'll work, but it won't hurt, so it's worth a try.