Secure Digital Memory Card (SD) - is a portable flash-memory card, developed for use in digital cameras, handheld computers, PDAs, and GPS units. It was developed by SanDisk, Panasonic and Toshiba based on previous MultiMediaCard (MMC)-card standard. The card has its own controller and protected area of memory, so it conforms to requirements of Secure Digital Music Initiative. The card provides a mechanism which prevents unauthorized reading of copyright protected data, hence the name Secure Digital. In most cases SD card could be replaced by MMC card in SD compatible devices, but MMC card may not be replaced by SD since most of then are physically thicker and may not be inserted in MMC slot. The capacity of SD card could be one of the following- 8MB, 16MB, 32MB, 64MB, 128MB, 256MB, 512MB, 1GB, 2GB, 4GB and 8GB. Speed of data exchange of SD cards is measured similar to CD-ROM and based on referral value 1X = 150KB/sec. Entry level models have speed of 6X (900KB/sec), the latest have 150X (22500KB/sec.)

MicroSD and MiniSD these two cards are just Secure Digital Memory Card in different form factor, developed for use in small devices. They may be used in any SD compatible device by the mean of simple physical adapter.

SDHC stands for Secure Digital High Capacity. It was developed as successor for SD standard for memory cards with 2GB and up capacity. SDHC cards can’t be used in devices which are not explicitly designed to support this standard.

CompactFlash one of the first formats of flash-memory cards was developed by SanDisk in 1994. It used in great number of digital devices. Although cards of newer standards offer higher data transfer rates and have smaller physical dimensions, CF cards still used widely in professional digital cameras. CF cards have many advantages compared to other standards one of them is excellent price per megabyte ratio.

xD-Picture Card stands for eXtreme Digital. It was developed by Olympus and Fujifilm in 2002. The only usage area of xD cards is digital camera products from Olympus and Fuji. It offers no advantages over other modern standards and typically costs nearly twice as much as cards of the same capacity from competitors.

Memory Stick is a standard of removable flash-memory devices introduced by Sony in 1998. It used primarily in digital consumer devices by Sony such as digital cameras, digital music players, PDAs, cellular phones, the PlayStation Portable (PSP), and in other devices.
Memory Stick PRO, Memory Stick Duo, Memory Stick PRO Duo, Memory Stick Micro (M2) are variations of Memory Stick cards. They differ from each other by data transfer speed and physical size.

MultiMediaCard (MMC) is a standard of flash-memory cards developed by SanDisk and Simens AG in 1997. Typically, an MMC is used as storage media for portable devices, such as cell phones and digital cameras.

External Hard Drive is just normal HDD enclosed in special plastic or metal case, also known as mobile driver case, and connected to PC with cable over USB (SCSI, IEEE 1394) interface. The main advantage of mobile HDD drives over flash-memory cards is its capacity- modern models has 500GB or even more space to store your digitized data. But due to its mechanical nature mobile drives are not well shock-protected and should be handled with care.