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Buyers Guides > Getting the most from your SD Memory Card

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The SD or "Secured Digital" memory card format was originally developed by Panasonic, SanDisk and Toshiba, and introduced in 2001 as a successor to the MMC or "Multimedia Card" format, which had been available since 1997. Indeed, the standard SD memory card has almost the same form factor (24mm x 32mm x 2.1 mm) as its predecessor, but is slightly thicker, features a write protection switch and wiring that allows DRM or "Digital Rights Management" to be enabled. This latter feature was originally introduced in response to concerns expressed by the music industry with regard to piracy, but as DRM has been roundly criticised on the grounds that it limits creativity, and allows rights holders to reserve rights to which they are not legally entitled, this capability is rarely used nowadays.

SD Memory Card Features, Tips & Techniques

The SD memory card is universally compatible with a wide range of portable digital devices, including digital cameras and cameras, mobile phones, multimedia players and PDA ("Personal Digital Assistant") devices. The standard SD memory card is typically available in storage capacities between 16MB; sufficient for storing 40 or so digital photographs, at reasonable resolution and often supplied "as standard" with digital cameras, and 2GB. The standard SD memory card also offers data transfer speeds of up to 900KBps ("Kilobytes per second"), or roughly six times the speed of a standard CD.

Most compatible digital devices can use SD memory cards up to 2GB in storage capacity, but do bear in mind that some of the latest digital cameras etc. are only compatible with the later SDHC or "Secure Digital High Capacity" card. The SHDC specification requires a storage capacity of 4GB, and classifies data transfer speeds; "Class 2" indicating a minimum, sustained, data transfer speed of 2MBps ("Megabytes per second"), "Class 4" 4MBps and "Class 6" 6MBps, although maximum read speeds are typically faster than maximum write speeds. Class 4 SDHC memory cards, with storage capacities of up to 8GB are already available, and the world's first 32GB, Class 6, SHDC card (capable of storing up to 8 hours of HD video) has been demonstrated, in prototype form. Furthermore, SDIO or "Secure Digital Input Output", is an interface that extends the functionality of SD devices, and SD cards are being developed for Blueooth®, GPS ("Global Positioning System") and 802.11 wireless applications.

The range of products that are incorporate SD card slots is growing rapidly. The Nintendo Wii and Sony Playstation 3 games consoles, for example, now include SD card bays, which allow players to expand the internal flash memory, not only for saved games, but for network settings, video settings, etc. and, in the case of the Nintendo Wii, Wii Channels that make use of an SD memory card. In this context, an SD memory card can also be used to store MP3 music files, digital photographs and video clips, which can be played back in conjunction with some games, or independently, on your television screen. Do be aware however, that many SD memory cards marketed as "gaming" memory cards are more expensive than standard SD memory cards of the same brand, despite being identical in specification and performance.

Smart phones, including those running the "Windows Mobile" operating system, now support SD memory cards, in one form or another. An SD memory card (whether it is a standard, full sized, SD card, or a smaller, "mini", or "micro", SD card) can be a fast and convenient method of transferring between a smart phone and a computer, for example, although a memory card reader or adaptor, may be required.

It is not uncommon however, for problems to occur when an SD memory card is inserted into a smart phone, or similar device. These may include an SD memory card suddenly "disappearing" (that is, no longer being recognised by a device) exhibiting symptoms of corruption (reporting no free space, or, conversely, no files, or folders) or simply "hanging" when an application or device, attempts to access the memory card.

Many such problems can be resolved by ejecting an SD memory card from its slot, and inserting it once again, so that a device has another opportunity to detect, and initialise, the card, making it visible once more. It is however, also possible for problems of this type to be caused by incompatibility between the chipset configuration of an SD memory card and a card reader, or the ROM ("Read Only Memory") of the device, itself. Japanese made SD memory cards, which feature a Toshiba chipset, are reported to be more compatible with Windows Mobile smart phones, for example, but if you do encounter this problem, an upgrade of firmware (computer chips that have data, or programs recorded on them) operating system, ROM, or a replacement SD memory card, is likely to be necessary.

You should also be aware that there are two possibilities for file systems on SD memory cards; FAT16 and FAT32. The choice of file system and "cluster" or "allocation unit", size (the minimum unit of file space on an SD memory card) can have a direct impact on the performance of an SD card, and the total volume of data that can be stored. FAT-based file systems almost always allocate more storage space for a file than is necessary. If for example, you store a 1kb file on an SD card with a cluster size of 16kb, you are effectively wasting 15kb of storage space, and the same is true if you store a 17kb file, because this requires two allocation units, and a total of 32kb. If you format your SD memory card to have the smallest cluster size possible (typically 512 bytes) such unnecessary wastage can be avoided; you can, for example, store a single byte of data in a file, and only waste 511 bytes of storage space.

The fact that the data on an SD memory card can be erased quickly and easily, and the storage space reused (indeed, the entire memory card can be reformatted if desired) is an obvious advantage when it comes to portable devices such as digital cameras and camcorders. By the same token, however, this does mean that SD memory cards are also susceptible to unwanted deletion or reformatting (accidentally, or maliciously) or errors introduced by systems failure, fire or water damage, etc.

If you find that the data on an SD memory card has been deleted by mistake, or is corrupted, the first thing to remember is the "Don't Panic!" maxim, of "Hitchhikers' Guide to the Galaxy" fame. Do not, for example, attempt to take any more digital photographs or video using the affected memory card, or attempt to reformat the card. Either action may result in the space on the SD memory card being overwritten, such that its contents become totally irrecoverable. You should, instead, move the write protect tab to the protected position, so that no data can be written to the card, before considering your data recovery options.

There is any number of software utilities available for recovering deleted or corrupted data from SD memory cards; some of these are freeware, or shareware, and some are commercial products. Results do however, tend to vary greatly from product to product, and SD card to SD card, so it is worth seeking out independent reviews of software utilities to determine which, if any, produce reliable results with your particular SD memory card, and in what circumstances. If your lost data is especially valuable, or irreplaceable, you may also like to consider sending your SD memory card to a data recovery service. Some services attempt recovery of your data on a fixed cost (typically £30, or so) and no recovery, no charge basis.