SD Filesystem

The 3DS uses a 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

All "extra data" under extdata is encrypted.

"Private" data is stored here:

/Nintendo 3DS/Private/<SomeID>/

00020500 - Nintendo 3DS Sound
00020400 - Nintendo 3DS Camera

"Private" data for 3DS Sound/Camera are cleartext. Under the camera priv dir is phtcache.bin, this seems to list the pictures on SD card? Under the sound priv dir is: voice/XX/*.m4a. Where XX is 01-10, with sound saved as .m4a.