The 3DS uses an SD Card for general storage of additional game data, music and photos taken with the 3DS.
/DCIM - Photos and images downloaded with the Internet Browser /Music - Music Files /Nintendo 3DS - Game Data
Additional game data is stored here:
/Nintendo 3DS/<SomeID>/<SomeID>/extdata/00000000
0000008f - Some 2.0.0-2 data, unknown doesn't appear in extdata management. 00000207 - Mii Maker - JPN 0000020d - Face Raiders - JPN 00000210 - Some 2.0.0-2 data, unknown doesn't appear in extdata management. 00000217 - Mii Maker - USA 00000219 - eShop USA 0000021d - Face Raiders - USA 00000227 - Mii Maker - EUR 0000022d - Face Raiders - EUR 0000030c - Nintendogs + Cats - EUR 0000032d - Super Street Fighter IV 3D - USA 0000033b - Ridge Racer 3D - EUR 0000033c - Super Street Fighter IV 3D - EUR 0000034d - Samurai Warriors Chronicles - USA 00000358 - Ridge Racer 3D - USA
All "extra data" under extdata is encrypted. Although these files use 0xFF blocks similar to FLASH saves, extdata can't be decrypted with the xorpad fail like FLASH saves. All "extra data" files can't be copied to other 3DS SD cards, they are locked to the console.
With the introduction of the June update the folder structure changed slightly. You will now find "dbs" and "title" folders located in /Nintendo 3DS/<SomeID>/<SomeID>/ along with the extdata folder. "dbs" contains two files, import.db and title.db - usage currently unknown.
/title/00040000/ appears to contain eshop downloads (can someone verify these and add for different regions?): 00054e00 - 3D Classics Excitebike - EUR (verified) 00054300 - 3D Classics Excitebike - USA 00032600 - Pokedex 3D - EUR (verified) For more IDs, see the 00040000 titles on the Title_list.
The above title directories contain two dirs: content and data. content contains 00000000.tmd, .app files, and some cmd dir containing 00000001.cmd, all of which are encrypted with a console-unique key. The data dir contains 00000001.sav, this is the title's encrypted savegame. Although these saves look similar to FLASH savegames, these savegames use proper unique CTR for each AES block in the file, and the CTR properly changes for each savegame write.
"Private" data is stored here:
/Nintendo 3DS/Private/<Title ID Low>/
00020400 - Nintendo 3DS Camera 00020500 - Nintendo 3DS Sound
"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.