Hi expatmanxman,
I use a Zumo 550 and while I've never had one break, I reckon that unless you loosen off the security screw, near fully unwind it, then you'll probably experience difficulty in locating the unit properly next time you mount.
My brother in law (has one too) was trying to wreck his one day, till I spotted him and loosened the screw. All brute force muttering that there was summat wrong, it won't fit.
Sometimes, when I've loosened the security screw to get the unit out, it's undone just enough to allow the unit to be removed, but impossible to get it back in again when I return, unless I unwind it a bit more.Dunno if you may be doing the same? May be worth considering? Hope it gets sorted.