SD card
Secure Digital cards, more commonly referred to as SD cards, are a digital storage medium. Initially primarily used for digital cameras, the media has since attained widespread use as a storage medium, and it has since been used on the Wii, DSi and 3DS as a method of storage expansion. The Wii U also supports SD cards, but only uses them when running Wii games.
In Super Smash Bros. Brawl
Standard save data for Brawl is not saved to SD cards, and instead, is stored on the Wii's own flash-based memory. SD cards, however, can be used to save snapshots, custom stages and replays from the Vault in order to be used or otherwise transferred on other Wii consoles. Of note is that because of the timing of the release of Brawl, the game does not support SDHC cards, a special type of SD card that supports over 2 gigabytes of data; while the Wii was later updated to take advantage of these cards, Brawl itself cannot be natively updated to do so.
Use in hacking
Through the use of an exploit in Brawl's Stage Builder, it is possible to load customised or otherwise hacked data from an SD card. In addition to allowing for the download of The Homebrew Channel, this also allows for the loading of replacement files, as well as gameplay mods such as Project M and Balanced Brawl. Through the use of the hack of Riivolution, it is also possible to force Brawl to read SDHC cards.
In Super Smash Bros. 4
The Nintendo 3DS has the ability to save downloaded software and various other types of data to an SD card, and the console even comes packaged with a two gigabyte SD card (3DS XL's and 2DS' come with a four gigabyte card instead.) It is, however, unknown if the 3DS version of Super Smash Bros. 4 will allow players to save data to the SD card, and if so, what specific data.
