Thanks catymag for your precious information, but I have some more questions if you permit me:

a) What do you mean "tiles limit" in "by Some old devices as 370 cant show maps greater than 2gb and have a tiles limit per map"?
b) It seems that GMapTools are better than (?)MapSource(?) - what else can be used to combine/merge maps - advantages/disadvantages?
c) I see there are maps with the "MapSource" identifier and other with no mapsource - what is the difference? Can both be used in a Garmin Nuvi or the 1st ones are for processing (w/MapSource maybe) and the second ones are *.img files to be used in the Nuvi?
d) I assume that for NUVI 370 I have to delete some "not-to-be visited" countries from the "CN EUrope NT 2011.40" to make it less than 2GB i.e. readable by the 370. What tool do you suggest me to use for this? Reminder: I need route-able and searchable maps.
e) CRITICAL Question: If I backup the NUVI to the HDD of my Pc or on a CD-ROM and something goes wrong someday (sooner or later), will the unit recover by "just copying" the backup files on the internal memory (i.e. Is there a need for a special bootstrap program located perhaps at a special/critical position in the internal memory, which with the "simple copying over"-procedure will not be able to recover the system?
f) What do you suggest for Backup-Recovery procedure apart from "DO NOT TOUCH YOUR INTERNAL MEMORY!!!" because I plan to ... risk to touch it!!!

Thanks in advance - GeorgeInParis