I have a 550T for which I bought GB Discoverer 1:50K ........... and then got another 550 (non T) ... (we both geocache) .. copied the GMAPSUPP.IMG to a new 4G Miscro SDHC and built a new GMAPSUPP.UNL for second GPS using unit ID and Garmin_kgen_1.5+Fix.exe worked perfectly .. thanks