SD Filesystem

The 3DS uses an SD Card for general storage of additional game data and photos taken with the 3DS.

/DCIM - Photos
/Nintendo 3DS - Game Data

Additional game data is stored here:

/Nintendo 3DS/<SomeID>/<SomeID>/extdata/00000000

0000034d - Samurai Warriors Chronicles - USA
0000033b - Ridge Racer 3D - EUR
00000358 - Ridge Racer 3D - USA
0000022d - Face Raiders - EUR
00000227 - Mii Plaza - EUR