File system

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File system

Post  kosovohp on Thu Sep 30, 2010 11:47 am

Like other flash card technologies, most SD cards ship preformatted with the FAT or FAT 32 file system on top of an MBR partition scheme. The ubiquity of this file system allows the card to be accessed on virtually any host device with an SD reader. Also, standard FAT maintenance utilities (e.g., SCANDISK) can be used to repair or retrieve corrupted data. However, because the card appears as a removable hard drive to the host system, the card can be reformatted to any file system supported by the operating system. Conversely, an SD card can contain an embedded operating system (such as a Live USB) to recover a corrupted host computer by natively booting from the flash media reader.

SD cards with 4 GB and smaller capacities can be used with many systems by being formatted with FAT16 (4 GB only possible by using 64 kiB clusters, and not widely supported) or FAT32 file system (common for file systems 4 GB and larger). Cards 4 GB and larger can only be formatted with a file system that can handle these larger storage sizes, such as FAT32.

SD cards are plain block devices and do not in any way imply any specific partition layout or file system thus partition schemes other than MBR partitioning and the FAT file systems can be used. Under Unix-like operating systems such as Linux or FreeBSD, SD cards can be formatted using, for example, the UFS, EXT3 or the ReiserFS file systems; under Mac OS X, SD cards can be partitioned as GUID devices and formatted with the HFS+ file system. Under MS-Windows and some unix systems, SD cards can be formatted using the NTFS and on later versions exFAT file system. However most consumer products will expect MBR partitioning and FAT16/FAT32 filesystem.

Fragmentation may slow down the effective write speed:[7] Defragmentation tools may be used. However, it is unnecessary to use any disk optimization tool because on an SD card the time required to access any block is the same. Defragmenting an SD card will wear the card out slightly, as the number of writes, before failure occurs, is limited (usually 100,000 times).

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