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http://en.wikipedia.org/wiki/Secure_Digital
File system
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.
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 kByte 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 other partition schemes 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.
Defragmentation tools are used on hard disks with physical discs to optimize the file system access speed by optimizing for physical disc and head movements. On an SD card, this is unnecessary, as 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 are limited before failure occurs (usually 100 000 times).
Speeds
There are different speed grades available, measured the same as CD-ROMs, in multiples of 150 kB/s (1x = 150 kB/s). Basic cards transfer data up to six times (6x) the data rate of the standard CD-ROM speed (900 kB/s vs. 150 kB/s). High-Capacity cards are made with higher data transfer rates like 66x (10 MB/s), and high-end cards have speeds of 200x or higher. SanDisk classifies their cards as:
Ultra II — minimum read speed of 15 MByte/s (100x)
Extreme III — maximum speed of 30 MByte/s (200x) (SanDisk Class 6)
Extreme IV — up to 45 MByte/s (300x) (Currently only available for Compact Flash formats)
Note that maximum read speed and maximum write speed may be different. Maximum write speed typically is lower than maximum read speed. Some digital cameras require high-speed cards (write speed) to record video smoothly or capture multiple still photographs in rapid succession. This requires a certain sustained speed, or the video stops recording. For recording, a high maximum speed with a low sustained speed is no better than a low speed card. The 2.0 specification defines speeds up to 200x.
Some manufacturers use the read speed in their X-ratings, while others (Kingston, for example) use write speed.[7]
This table lists common ratings and minimum transfer rates.
Rating Speed (MByte/s) SD Class
6x---------------0.9----------n/a
10x--------------1.5----------n/a
13x--------------2.0----------2
26x--------------4.0----------4
32x--------------4.8----------4
40x--------------6.0----------6
66x--------------10.0---------10
100x-------------15.0---------15
133x-------------20.0---------20
150x-------------22.5---------22
200x-------------30.0---------30
266x-------------40.0---------40
300x-------------45.0---------45
SD Speed Class Ratings
SD Cards and SDHC Cards have Speed Class Ratings defined by the SD Association. The SD Speed Class Ratings specify the following minimum write speeds based on "the best fragmented state where no memory unit is occupied":[8]
Class 2: 2 MByte/s - 13x
Class 4: 4 MByte/s - 26x
Class 6: 6 MByte/s - 40x
SD and SDHC cards will often also advertise a maximum speed (such as 133x or 150x) in addition to this minimum Speed Class Rating. Important differences between the Speed Class and the traditional "X" speed ratings are; 1) the ability of the host device to query the SD card for the speed class and determine the best location to store data that meets the performance required, 2) class speed defines the minimum transfer speed. Even though the class ratings are defined by a governing body, like "X" speed ratings, class speed ratings are quoted by the manufacturers but unverified by any independent evaluation process.
On 21 May 2009, Panasonic announced new "class 10" SDHC cards, claiming that this new class is "part of SD Card Specification Ver.3.0".[9] Toshiba also announced cards based on the new 3.0 spec[10] As of November 2009[update], the SD Association's Web site does not include information on this new class or new specification.
