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The MultiMediaCard, in the form of the SD card (described below), has become the most popular removable storage system. The SD card has superseded the MultiMediaCard, although some MMC cards can still be read in SD card slots.

Secure Digital (SD)

Secure Digital is the descendant of the MultiMediaCard and is now the most popular format for removable storage.

Sizes

SD cards come in three form factors: Standard, Mini, and Micro. Standard is the only one with a write-protect tab. The Mini SD Card is the least common and is no longer manufactured. The Micro SD card is also called TransFlash, T-Flash or TF. Many computers and other devices come with a Micro SD card slot instead of a standard one. Micro SD cards often come packaged with an adapter to fit into a standard slot.

Capacity Class

Capacity Class		Maximum Capacity
SDSC	Standard Capacity	4 GB*
SDHC	High Capacity	32 GB^†
SDXC	Extended Capacity	2 TB
SDUC	Ultra Capacity	128 TB

* 4 GB SD Cards may be SDSC or SDHC.
†� Older devices or card readers may be unable to read SDHC or higher capacity cards.

If the card complies with the UHS speed class specification, modern devices can warn if a card that performs too slowly for the device is inserted.

Speed Classes

Speed Class	Minimum Speed
Class 2	2 MBps
Class 4	4 MBps
Class 6	6 MBps
Class 10	10 MBps-plus
UHS-I	104 MBps*
UHS-II	312 MBps^†
UHS-III	624 MBps^‡
SD Express	3938 MBps^�

* Up to 10 MBps (theoretically up to 50 MBps). Up to 104 MBps with UHS104.^†Theoretically, up to 312 MBps half-duplex^‡Theoretically, up to 624 MBps half-duplex
� Theoretically, up to 3938 MBps full-duplex using PCIe 4.0 x2

UHS-II and UHS-III have a second row of pins and only work at full speed in connectors that interface with the second row

If the card complies with the UHS speed class specification, modern devices can warn if a card that performs too slowly for the device is inserted.

SD Express

SD Express was released in 2018. It uses a single PCIe lane and the NVMe architecture with transfer rates up to 3.93 Gbps. Like UHS-II and UHS-III, SD Express cards have two or three rows of pins.

SDIO

Secure Digital Input Output (SDIO) supports several I/O devices in the SD Card from factor. Such devices include GPS receivers, barcode readers, FM radio tuners, WiFi, Bluetooth, etc.

Caveats:

Watch for fake SD cards on eBay, Amazon, etc. Cards can be hacked to say (to the computer) that they have more capacity than they actually do. They are then relabeled and repackaged to look like the real deal. You don't find out until you exceed the actual capacity of the card when older data is overwritten (and it's too late to return the card).

Amazon and eBay quickly remove the listings for such cards, but the sellers just open new accounts and relist them under a new name. These cards are usually branded with strange names but can be labeled with trusted brands. You can't trust the reviews because the company first stuffs the listing with fake five-star reviews before the listing gets removed.

MultiMediaCard and Secure Digital