Mass Storage Basics - USB Mass Storage: Designing and Programming Devices and Embedded Hosts - page 13

Hard Drives
When you need a lot of storage, a hard drive is the most economical choice.
At this writing, a megabyte of hard-drive storage is 50 to 100 times cheaper
than a megabyte in a flash-memory card. Prices for both media continue to
fall, and the price differential may change over time, but for the near future,
hard drives are likely to continue to be the favored solution for storing very
large amounts of data.
Technology
Because they use mechanical drive components, hard drives tend to be more
fragile than completely electronic media such as flash memory. Modern
drives are much more rugged than in the past, however. For embedded sys-
tems that need to fit in a small space, tiny hard drives are available in the
USB key-drive form factor and in Type II CompactFlash cards.
Interfaces
The most common interface between a hard drive and its CPU is the Paral-
lel AT Attachment (ATA) interface, also known as the Integrated Drive Elec-
tronics (IDE) interface. A drive that uses ATA must have an intelligent
controller embedded in the unit. The ATA specification defines the cables
and connectors, signals, and registers and commands for communicating
with the drive's controller. ATA devices must support logical block address-
ing. A single ATA interface on a host computer can connect to up to two
storage devices. The host computer communicates by reading and writing to
registers in the device.
ATA with Packet Interface (ATAPI) is an extension to ATA that defines a
protocol for sending SCSI and other commands to an ATA device in struc-
tures called command packets. CD and DVD drives use the ATAPI proto-
col. More information about ATA/ATAPI and links to the standard
documents are at www.ncits.org.
Flash Memory
Flash memory is non-volatile, electrically erasable storage available as chips
and in cards that incorporate memory chips and a controller.