Hi - We have a Zumo 550 which is hard wired into a Harley Davidson. At the moment it is frozen and will not turn off. It is at present somewhere between Turin, Italy and Chambery, France heading to a hotel in Chambery (if we can rebott it)
The screw on the battery is actually a #8 torx.
I am having this problem too, the power button does not seem to work. Freezes on and freezes off.
Mine is hard wired into a KTM 990 and is powered all the time.
Removing the Zumo from the powered mount asks if you want to shut it off.
Removing the battery clears the frozen off.