here is a link for a replacement security screw for the Zumo 550. it has a knurled end which allows you to easily loosen it up to allow removing the GPS. clearly wouldn't prevent a thief from stealing the GPS, but it is the safety (security) screw that ensure the GPS doesn't accidently come unlatched from the motorcycle mount. was pretty cheap, so I ordered one plus a spare. kinda liked the clean appearance rather than using velcro or some other fabricated lock.


http://www.st-owners.com/forums/showthread.php?t=49002

 

I have both the knurled screw and lever replacement, and I like the lever better. I did a write-up here with photos. Neither is as secure as the stock screw that requires the special screwdriver, but none are truly safe against theft, and unless you have the lock kit for the Ram mount a thief must only cut the wires and unscrew it to be off with your GPS.

Although I rarely remove my Zumo, usually using it as a helmet rack while parked to hide its presence, I found the stock screw very tedious to remove. If you made a habit of removing it frequently, or even if not, I would definitely go with either of the two aftermarket alternatives.

 

I've got the thumb screw. Makes it a breeze to take the Zumo off when leaving the bike.

That lever looks neat too.