The Nuvi uses a disk based file system and can support multiple maps on the hard drive and the SD card.
You can have various mapsets loaded on your nuvi...not just the basemap and detail map. These mapsets all reside in the x:/Garmin folder on both the nuvi drive and the SD Card drive. The nuvi 7x0 can access up to six different mapsets (*one* on the SD Card). The nuvi 7x5 can access up to nine different mapsets (*one* on the SD Card)
Mapset 1. City Navigator North America NT 2009 = nuvi drive main detail mapset (gmapprom.img)
Mapset 2. City Navigator Mexico NT 2008 = nuvi drive OEM detailed mapset (gmapoem.img)
Mapset 3. US Topo 2008 = nuvi drive supplemental detailed mapset (gmapsupp.img)
Mapset 4. City Navigator Europe NT 2009 = SD Card drive supplemental mapset (gmapsupp.img)
Mapset 5. nuvi drive Basemap (gmapbmap.img)
*Note: Before fiddling with mapsets, make a backup of them first, especially gmapprom.img as this is your main detailed mapset.
Here's a list of the nuvi 7x0 & 7x5's mapset names & their relationship with the nuvi
5. gmapoem.img: (7x0/7x5) nuvi drive only (OEM mapset), user accessible, cannot be unselected from the unit via Tools/Settings/Map/Map Info
*Note, when using the following default system maps as user maps, you'll naturally lose their default map attributes.
6. gmapbmap.img: (7x0/7x5) nuvi drive only (default basemap), configurable as a supplemental map; rename desired mapset to gmapbmap.img, NOT selectable and does NOT show up under installed maps (Tools/Settings/Map/Map Info) but may be visible under Tools/Settings/System/About/Basemap
7. gmaptz.img: (7x5 only) nuvi drive only (default 7x5 Timezone map), configurable as a supplemental map; rename desired mapset to gmaptz.img
8. gmap3d.img: (7x5 only) nuvi drive only (default 7x5 3D Buildings map), configurable as a supplemental map; rename desired mapset to gmap3d.img
+9. gmap3d1.img (7x5 only) nuvi drive only (secondary 7x5 3D Buildings map), configurable as a supplemental map; rename desired mapset to gmap3d1.img
Nuvi internal (diagnostic utility) OS map naming
885T File structure has been changed from the 7x5 series
Interesting. The 885T I just received does not recognize the gmapprom1.img file name like my 765T does. Called Garmin and they said is should, but I will be darned if anything, locked or unlocked shows up under that moniker... However, the gmapoem.img name does still work and it does show up under Map Info and it is, selectable. Just goes to show things do change in the Garmin world. lol