SD Filesystem

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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

/DCIM with 3.0.0-5 also stores .avi 3D videos from the camera title, video frames use MJPG.

Extdata

Additional game data is stored here:

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

See the extdata page for more extdata info and the extdataIDs list.

All "extra data" under extdata is encrypted. Extdata can't be decrypted with the xorpad fail used for old FLASH saves. All "extra data" files can't be copied to other 3DS SD cards, they are locked to the console.

import.db and title.db

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. The purpose of the files in the "dbs" is to archive the list of titles installed on the SD card. These two files are only used for the management of SDMC installed 3DS Titles. The "title.db" file, archives the installed titles aswell as their, names, icons and size. The "import.db" also changes when a title is installed or deleted, but doesn't appear to contain info that the titles "title.db" does. Old and new of the "import.db" and "title.db" can be interchanged without any issues. Essentially the "title.db" acts as the DRM or access rights for downloaded SD card titles.

title.db seems to be encrypted.

title

/title/ Contains eShop title downloads
00040000/00032600 - Pokedex 3D - EUR 
00040000/00042a00 - Legend of Zelda - Link's Awakening - EUR
00040000/0004ab00 - Nintendo Video - EUR
00040000/00052000 - Let's Golf 3D - EUR
00040000/00054300 - 3D Classics Excitebike - USA
00040000/00054e00 - 3D Classics Excitebike - EUR
00040000/00045C00 - 3D Classics Excitebike - JPN
For more IDs, see the 00040000 titles on the Title_list.

Standard Directory Structure for SDMC Installed Titles:

/title/0004XXXX/XXXXXXXX/Content/00000000.tmd
                                /XXXXXXXX.app
                                /cmd/00000001.cmd
/title/0004XXXX/XXXXXXXX/Data/00000001.sav
   
                                                

"00000000.tmd" - This is the Title Metadata associated with the title, it is encrypted with a per-console key. The decrypted TMD is available on Nintendo's CDN server at "http://nus.cdn.c.shop.nintendowifi.net/ccs/download/TitleIDhere/tmd". Though CDN version of the title TMD has a certificate chain attached at the end of the TMD, so removing it will give you the 1:1 decrypted TMD. After title installation, the important information in TMD is likely stored somewhere, as modifying/deleteing the TMD file, doesn't usually have any effect.


"XXXXXXXX.app" - These files are NCCH files, where the entire file is encrypted with a per-console key. There can be more than one NCCH in this directory, as seen with .CCI files, the game executable (CXI) can be accompanied with additionally non-executable NCCH files (CFA) such as the game manual and DLP Child containers. Determining the contents of the encrypted NCCH, is done by finding the Content Index of the "XXXXXXXX.app" file in the title's TMD(see above for retrieving decrypted TMD), interpreting the Content Index is as follows:

Index Content Type
0000 Main Executable (.CXI)
0001 Home Menu Manual (.CFA)
0002 DLP Child Container (.CFA)

Unlike the TMD, a decrypted version of the NCCH files cannot be retrieved from Nintendo's CDN, the NCCH files do exist on Nintendo's CDN but are encrypted.Of course editing/deleting ".app" files will have an effect. Deleting/renaming the manual ".app' will cause the manual not to load when clicked on. And deleting/renaming the executable ".app" will cause the application to not load, and the 3D Banner does not show(The banner is loaded each time from the game's executable NCCH when the home menu loads, it is not cached like the icon and name).


"00000001.cmd" - (in some case the file is called "00000002.cmd") May contain hashes, or some other kind of file authentication, as the size of the "00000001.cmd" varies with the number of files in the 'Content' directory. Also, if the "00000001.cmd" cannot be found or is edited, the title is completely non-functional.


"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. Renaming these savegames causes home-menu to hang while launching titles, modifying saves throws the usual checksum/hash corruption like gamecard flash saves.


Private

"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? When you want to install and see pictures with 3DS,rename to 8 numbers.mpo and save it on /DCIM . Under the sound priv dir is: voice/XX/*.m4a. Where XX is 01-10, with sound saved as .m4a.