My original 3760 came with just 1.7 Gb internal memory.
Since I transformed 3760 to 3490 I use the following settings for the last six months:

Internal memory: \.System\SQLite\
Internal memory: \.System\ASR\ with only .srd files of the different languages supported (like de_DE.srd, de_US.srd, de_GB.srd etc) updateable by WebUpdater.

sdCard:\Garmin\ASR\ with .srx and .asr files, that are updated every time CNEUNT is updated.
sdCard:\Garmin\JCV\ with .jcv file, that is updated every time CNEUNT is updated.
sdCard:\Garmin\SID\ with .sid files, that are updated every time CNEUNT is updated.

I was writing Gmapprom.img (CNEUNT) in sdCard:\MAP\ folder, but lately Garmin changed that and 3490 can now read any map, with any name, from sdCard:\Garmin\ folder.

If you use a Marine Chart (G2, or G2 Vision) that should be in sdCard:\Garmin\ folder, named as Gmapsupp.img, so that HomePort can read it.

No .System folder in sdCard.