Just purchased a new zumo 550. Serial number is required to register, but is under the battery. Problem is, cover will not unscrew! Best part is the company of sales offered to give me the serial number.....ya, but it is a new unit and should not be a defect; just mailed it back. Just want to bring this to peoples attention. If it was not for the serial number to register, I would have had it for a long time before trying to open. Cover screw turns CCW forever, but cw a 1/4 turn and it binds up.