Secure Digital card

An SDHC card, with 8 GB capacity; this one is class 6. This means it has a minimum data rate of 6 MB/s

A Secure Digital card (SD card) is a kind of memory card. Such cards are often used to store images or data in digital cameras. As of 2008, different capacities between 4 megabytes and 32 gigabytes have been made. The card has a rectangular design, but one edge is chipped off. This means that the cards cannot be inserted into the cameras (or other devices) the wrong way.

There are different kinds of cards:

• Those labeled SD, with capacities up to 2 GB
• Those labeled SDHC, with capacities between 4GB and 32GB
• Those labeled SDXC, with capacities of up to 2 TB (largest made is currently 512GB for SDXC or microSDXC).

SD and SDHC are not compatible, but devices that accept SDHC also accept SD cards. The interface of SDHC and SDXC cards is the same, but SDXC uses a different file system. Some devices (for example the Wii) that originally shipped only with support for SD can be made to support SDHC with a firmware update.

There are also different classes. These refer to the read and write speeds. Currently in use are Class 2 - which means 2 MBytes/second, Class 4 - 4 MB/sec and Class 6 at 6 MB/sec. The problem with this is that different manufacturers measure different things with it, some measure write speeds, others read speeds. There are also differences because some give guarantees, and others simply say that the speeds they indicate are the maximum achievable under good conditions.

Still another measurement is called rating. A rating of 1 corresponds to 150 KBytes per second, the read speed of a CD. That way, a class 2 is equal to a speed of 13x.

microSD cards do the same thing but are much smaller.

Data recovery

A malfunctioning SD card can be repaired using specialized equipment, as long as the middle part, containing the flash storage, is not physically damaged. The controller can in this way be circumvented. This might be harder or even impossible in the case of monolithic card, where the controller resides on the same physical die.

File systems

The standard file system for SD and SDHC cards is FAT (up to and including 2 GB — FAT16, from 2 to 32 GB — FAT32), for SDXC cards (from 64 GB) — exFAT file system). Many manufacturers supply cards pre-formatted. However, Secure Digital cards can be formatted by yourself in any way you like with the appropriate software. The use of NTFS with the default settings is not desirable because the number of rewrite cycles for the card is limited. The most common reasons for protection:

• SD card is locked with a physical switch
• Write protection has been added or SD card is encrypted with a BitLocker password.
• SD card is protected against writing with viruses or unknown malware
• Сard is damaged or contains defective sectors.
• SD card is written and protected by third-party encryption software.

However, file system support varies depending on the operating system or firmware of the device using the card; for example, some devices support only FAT16, so the maximum card size is limited to 2GB. SD cards for navigation systems can have different formats.