common form factors prevailing today are 3 2 inch and 2 2 inch.
The form factor usually refer to the width of the drive enclosure or the
diameter of the disks used. However, in some cases, form factor represents
neither of the two. For example, the width of a 3 2 inch HDD enclosure is
4 inch and the disks used in these drives have diameter larger than 3.5 inch.
This particular form factor got its name from the fact that the size fi ts well in
the space originally allocated for 3.5inch fl oppy disk drive.
Phenomenal increase in areal density achieved over last few decades allows
the manufacturers to increase storage capacity with simultaneous decrease in
the size of hard disk drive suitable for applications such as laptop computers,
cameras, and other small devices. The trend in form factors is downward: to
smaller and smaller drives. The fi rst form factor used in a PC (5 4 inch) have
now all but disappeared from the mainstream PC market, and the 3 2 inch
form factor dominates the desktop and server segment. For laptop market,
the dominant form factor is 2 2 inch. HDDs of smaller form factor is the most
desirable choice for the emerging market of digital entertainment with devices
such as digital camera, MP3 etc. The micro drive of IBM is less than 0.25-inch
thick and uses disk one inch in diameter. Continuous growth of areal density
will initiate soon a transition to the 2 2 inch form factor for the desktop and
server drives. The reasons for this shrinking trend include the enhanced rigidity
of smaller platters, reduction of mass to enable faster spin speeds, and improved
reliability due to enhanced ease of manufacturing. Faster spin speed increases
therateofdatatransferbetweenthemedia and read/write electronics, reduces
latency, and therefore, improves data access performance. IBM's micro-drive
uses disks of only 0.85-inch diameter and can store suﬃcient data useful for
hand held and entertainment devices such camcorder, PDA, and portable MP3
players. The capacity of this drive was 2 Gbyte in 2004 and 4 Gbytes in 2005.
Toshiba announced their plan to introduce 8 Gbyte micro drives in 2006 which
will use perpendicular recording technology. Dimensions for drives of different
form factors are tabulated in Table 1.2.
Smaller form factor drives usually come with lower performance than a
larger drive. Spindle speed is usually lower in these drives, for example, IBM
micro drive uses 3600 RPM spindle motor. However, because of the small
size of disk(s), it can be spun up very fast. A small drive can spin up to full
speed in less than half a second. This makes it possible to spin down the drive
frequently, which is an essential feature for portable computers.
1.4.3 Trend in Data Transfer Rate
Another signi fi cant change that the HDD industry experienced in the past
is the trend in the access time and data transfer rate. For applications that
require faster data rates, speeding up the disk rotational speed has reduced
the latency component of access time and increased the speed of data fl ow
from the heads. There has been a steady progression over the years from