For img I think gimgunlock is easier, .unl should work with maps with FID less than 2200, structure is common new Nuvis series structure as shown in post n1 picture+ anyname.img in Map folder. gmap3d.img can be renamed in Map folder as you like, for example gmap3d_CNE.img or gmap3d_CNA.img: usually Nuvi recognizes it. You can put gmapprom.img in sd in Garmin folder too.
For additional files in SD: Everything is usually in internal mem. in .System should work in sd in Garmin folder:
sdCard:\Garmin\ASR\ with .srx and .asr files
sdCard:\Garmin\JCV\ with .jcv file
sdCard:\Garmin\SID\
etc.
You can find all these infos in
[Only registered and activated users can see links. ]
SP acts like a 3xxx series Nuvi