Secure Digital
Secure Digital (SD) is memory card that uses flash memory technology so there is no moving parts as seen with the HDD. It is a younger technology compared to those of HDDs and SSDs as they were first introduced in 1999 by Panasonic, Toshiba, and SanDisk. The device was created as a new format to replace many of the pre-existing types of memory cards such as the SmartMedia (SM) cards. The form factor of the SD cards dominated as they could hold large amounts of data in a tiny card that is also durable compared to the flimsy SM cards that were prone to bending.
SDs are really small in size and are used in everyday mobile devices such as GPS, cellular phones, digital cameras, music players, and even laptops have slots for them.
In their humble beginning, they only held about 128 MB of memory and around 2008 they were in the range of 2 GB to 8 GB of storage. As technology in the data storage field advanced, more storage space was allowed into the tiny device. Now, in 2014, we see SD cards that can go up to 128 GB of memory.
The emergence of the microSD cards maxed out at 64 GB and are more commonly seen used in newer phones such as smartphones and newer music players that allow for external storage.
An SD card is about 32 milimeters in length and a microSD card is about 11 milimeters in length. They are roughly the same in thickness. The SDs perform similary like HDDs and SDDs in that they can read and write at certain speeds and can be interchangeable/upgradeable. The rate of data accessing on SD cards ar ranked by the numbers 2, 4, 6 and 10 and are marked on the card. A class 10 SD card would have the fastest data accessing rate up to and not limited to 90 MB/s.
The faster SD cards are primarilary used for High Definition Media such as high definition recordings of audio, video, or picture. The SD cards are not limited to media storage but can also store documents and any other files such as the HDDs and SDDs.