External locks will only slow someone down, if at all, especially if it's not locked to a stationary object. If thieves want it badly enough, it'll be gone. My brother had his GSX-R750 parked underneath an overhead storage unit in an apartment complex parking space, with his car parked in front of it, with a u-lock thru the rear swingarm, and it still got stolen. Now if he had a cable and locked it to his car, that would've slowed them down, and might have even deterred them, but there's no way in knowing that now.
I'm kinda paranoid about leaving my bike in a public place where I can't see it, i.e. going to the movies. I have my bikes in a public storage facility, with a lock thru the front rotors, and a thick cable going thru all the wheels, essentially locking them all together. The lock on the rollup door has two locks on it as well. I don't lose sleep at night, thinking they won't be there next time I want to go riding, because I've done pretty much everything I could to keep them secure, plus there's insurance. But there's nothing like that "punch in the gut" feeling when something of yours gets stolen...
