Secure Digital (SD) and CompactFlash (CF) cards consist of flash memory and a controller. Flash memory is non-volatile meaning it retains data without external power. The memory in these cards is the NAND (Negated AND) type that is written and read in blocks rather than single bits at a time.
NAND flash memory stores information in individual cells by setting an electrical charge. A cell may contain either a charge or no charge ("0" or "1") in single-level type memory, or in multi-level cell type memory a specific charge level may be stored.
The second component of a memory card is the controller. This controller is essentially a small computer that acts as the interface between the flash memory and the host device (camera or card reader). It handles reading and writing, voltage regulation, and other tasks including error correction and bad sector management. The controller may also incorporate wear leveling techniques to extend the life of the card by monitoring and distributing data so certain areas of the card are not overused. The controller directly affects the transfer speed capabilities of a memory card.
While there are many brands of memory cards, very few manufacture the memory chips used inside the cards. Of the major memory card brands, only Samsung, SanDisk, Toshiba and Lexar are involved in memory chip manufacturing. Samsung is the leading producer of NAND memory worldwide and opened a new $7 billion facility in Xi'an, China to produce NAND flash memory in May, 2014. Toshiba and SanDisk co-own a memory production facility in Yokkaichi, Japan. Micron, owner of Lexar, produces memory chips in a joint venture with Intel at a facility in Lehi, Utah and assembles their cards in Asia. Other NAND memory chip manufacturers include TSMC in Taiwan and SK Hynix of South Korea.
Flash controllers for SD and CF cards are manufactured by Samsung, Micron, Sandisk, Toshiba and Sony as well as several independent manufacturers including Alcor Micro, JMicron, Phison, SiliconMotion (SMI) and Hyperstone.
Flash memory card brands not directly involved in production may use one or more of the above suppliers. Kingston is known for using Toshiba/SanDisk chips and controllers, Transcend has used Samsung chips and SMI-produced controllers, Delkin uses Toshiba, Samsung and Micron chips and custom designed controllers from various manufacrurers including Hyperstone. The controller is a vital part of a memory card; a well-established brand may work closely with a manufacturer on a custom controller design. Lesser known brands may pick and choose parts including generic controllers. Because these brands are not tied to any particular manufacturer, product specifications, performance and reliability may change over time.
There are numerous counterfeit SD and CF cards available and it is difficult for a consumer to physically detect a counterfeit. It is not apparent until the user experiences trouble: slow transfer speed, overstated capacity, corrupted data or a short lifespan. It is relatively easy for a counterfeiter to take a low capacity card and alter its firmware to report a larger capacity than is actually available on the card. When you exceed the actual capacity you lose data! Beyond losing pictures, the card manufacturer will not honor a warranty on a fake card. Because counterfeits are so prevalent you should purchase memory cards from an authorized dealer: B&H Photo, Adorama or Amazon (only direct from Amazon, not 3rd party sellers) or a trusted retailer. Memory cards from third party sellers are more likely to be suspect, especially sellers that ship direct from China.
Manufacturers take measures to guard against counterfeits. Many cards are printed or engraved with unique batch numbers. Some manufacturers add a holographic security label to the card and/or packaging. The card may also contain an internal identifier or serial number that can be read using the appropriate interface. Our card reviews include internal SD card information to help you determine the authenticity of your card.
The best way to test your SD or CF card is to fully write and read the card. To do this, use a program such as H2testw (Windows). It will detect any errors and find the true capacity of your card. Although your operating system may report the capacity of the card, this is only what the card controller says. You can only test the true capacity by writing data to the entire card.
CrystalDiskMark is often used to test read and write speeds. The program offers a variety of tests, you can run all or individual tests. When performing multiple passes of a test only the highest speed is reported.
The results of benchmark tests depend on your card reader, connection speed and computer. Just because a card tests slower than advertised does not automatically mean the card is bad or you have a fake. Benchmarks provide a simulation of card speed using specific criteria, whereas the acutal performance of a card used in a camera will differ. See our camera memory card speed tests for actual results of write speed tests as mesured in cameras.
You can format your card using your camera or computer. If you intend to take photos, format the card in your camera. If you want to format using your computer, you can use the official SD Formatter 4.0 program. It will format the SD card to specification and offers options including a full overwrite format. If you format your SD card using your operating system: use FAT32 for up to 32GB SDHC and exFAT for larger capacity SDXC cards.
If you get errors reading your SD card, you may still be able to recover your pictures. The file allocation table may be corrupted but the underlying data may still be intact. You can use a program to read data from the card to try to recover your files. PhotoRec and Zero Assumption Recovery are free tools to help recover data from corrupted SD cards. Even after a card is formatted, the raw data may still exist on the card. Normal formatting only removes the file system.
Warranty claims require a genuine product and may also require proof of purchase from the original owner (sales receipt). Below is a list of warranties offered by various flash memory card brands:
|Sandisk||Lifetime||Extreme Pro, Extreme Plus, Extreme and Ultra Plus|
|Lexar||Lifetime||Professional, Platinum, Platinum II and Premium cards|
|Samsung||10 years||EVO, PRO, Pro Series, Plus Series cards|
|5 years||Essential, High Speed and Standard Series SD cards|
|Kingston||Lifetime||Flash memory cards|
|Delkin||Lifetime||SD and CF cards|
|Toshiba||5 years||All memory cards|
|Sony||1 year||Secure Digital cards|
|5 years||CompactFlash cards|