The DSi uses a 256MB Samsung moviNAND flash chip, which is a NAND flash chip with a built-in controller that implements a MMC (SDIO) interface. In many ways, it's like an SD card in BGA packaging, and some people have successfully read it with modified SD card readers. The last 16MB is used for wear-leveling purposes (such as replacing bad blocks), while the first 240MB is used for storing actual data.
Addressing is done in terms of 512-bytes sectors. All wear-levelling and bad-block-mapping is handled transparently inside the chip by the controller. Most sectors are encrypted with a per-console key.
|0||0x200||PC-style Master Boot Record (MBR), encrypted with a per-console key|
|0x200||0x200||Stage 2 info block 1 (see stage2)|
|0x400||0x200||Stage 2 info block 2|
|0x600||0x200||Stage 2 info block 3|
|0x800||0x4DC00||Stage 2 bootloader (encrypted with a universal key)|
|0x4E400||0x400||Stage 2 footer -- unknown format, but first 10 bytes are build number of Stage 2 bootloader|
|0x4E800||?||Unused (all 00)|
|0xffa00||0x400||Diagnostic area. (often contains build date of device in plaintext) Blank in never-before-booted DSi. Might be written to during firmware updates.|
|0x10ee00||?||First sector of main FAT32 partition|
|0xcf09a00||?||First sector of photo FAT32 partition|