CompactFlash

CompactFlash (CF) was originally developed as a type of data storage device used in portable electronic devices. For storage, CompactFlash typically uses flash memory in a standardized enclosure. This form was first specified and produced by SanDisk in 1994. The physical format is now used for a variety of devices

Description
There are two main subdivisions of CF cards, Type I (3.3 mm thick) and the thicker Type II (CF2) cards (5 mm thick). The CF Type II slot is used by Microdrives and some other devices. There are four main speeds of cards including the original CF, CF High Speed (using CF+/CF2.0), a faster CF 3.0 standard and a yet faster CF 4.0 standard that is being adopted as of 2007. The thickness of the CF card type is dictated by the preceding PCMCIA card type standard.

CF was among the first flash memory standards to compete with the earlier and larger PC card Type I memory cards, and was originally built around Intel's NOR-based flash memory, though it switched over to NAND. CF is among the oldest and most successful formats, and has held on to a niche in the professional camera market especially well. It has benefited from having both a good cost to memory size ratio relative to other formats for much of its life, and generally having larger capacities available than smaller formats.

CF cards can be used directly in PC Card slot with a plug adapter, used as an IDE hard drive with a passive adapter, and with a reader, to any number of common ports like USB or FireWire. As it has a bigger size relative to the smaller cards that came later, many other formats can be used directly in a CF card slot with an adapter (including SD/MMC, Memory Stick Duo, xD-Picture Card in a Type I slot, and SmartMedia in a Type II slot, as of 2005) (some multi-card readers use CF for I/O as well).

Flash memory, regardless of format, supports only a limited number of erase/write cycles before a particular "sector" can no longer be written. Memory specifications generally allow 10,000[1] to 1,000,000 write cycles. Typically the controller in a CompactFlash attempts to prevent premature wearout of a sector by mapping the writes to various other sectors in the card - a process referred to as wear levelling.

Type I and Type II
The only difference between the two types is that the Type II devices are 5mm thick while Type I devices are 3.3mm thick.[3] The vast majority of all Type II devices are Microdrives and other miniature hard drives. Flash based Type II devices are rare but a few examples do exist. [4][5] Even the largest capacity cards commonly available are Type I cards and most card readers will read both formats with the exception of some early CF based cameras where the slot is too small and some of the poorer quality USB card readers with the same problem.