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6.1 / MAGNETIC DISK 191

platter, allowing an air gap. At the other extreme is a head mechanism that actually
comes into physical contact with the medium during a read or write operation. This
mechanism is used with the floppy disk, which is a small, flexible platter and the
least expensive type of disk.
To understand the third type of disk, we need to comment on the relationship
between data density and the size of the air gap. The head must generate or sense
an electromagnetic field of sufficient magnitude to write and read properly. The
narrower the head is, the closer it must be to the platter surface to function. A nar-
rower head means narrower tracks and therefore greater data density, which is de-
sirable. However, the closer the head is to the disk, the greater the risk of error
from impurities or imperfections. To push the technology further, the Winchester
disk was developed. Winchester heads are used in sealed drive assemblies that are
almost free of contaminants. They are designed to operate closer to the disk’s sur-
face than conventional rigid disk heads, thus allowing greater data density. The
head is actually an aerodynamic foil that rests lightly on the platter’s surface when
the disk is motionless. The air pressure generated by a spinning disk is enough to
make the foil rise above the surface. The resulting noncontact system can be engi-
neered to use narrower heads that operate closer to the platter’s surface than con-
ventional rigid disk heads. 1
Table 6.2 gives disk parameters for typical contemporary high-performance
disks.

Table 6.2 Typical Hard Disk Drive Parameters
Seagate Seagate Seagate Hitachi
Barracuda Barracuda Barracuda Micro-
Characteristics ES.2 7200.10 7200.9 Seagate drive

Application High-capacity High-performance Entry-level Laptop Handheld
server desktop desktop devices
Capacity 1 TB 750 GB 160 GB 120 GB 8 GB
Minimum track-to-track 0.8 ms 0.3 ms 1.0 ms — 1.0 ms
seek time
Average seek time 8.5 ms 3.6 ms 9.5 ms 12.5 ms 12 ms
Spindle speed 7200 rpm 7200 rpm 7200 5400 rpm 3600 rpm
Average rotational delay 4.16 ms 4.16 ms 4.17 ms 5.6 ms 8.33 ms
Maximum transfer rate 3 GB/s 300 MB/s 300 MB/s 150 MB/s 10 MB/s
Bytes per sector 512 512 512 512 512
Tracks per cylinder (num- 8 8 2 8 2
ber of platter surfaces)

1 As a matter of historical interest, the term Winchester was originally used by IBM as a code name for the
3340 disk model prior to its announcement. The 3340 was a removable disk pack with the heads sealed
within the pack.The term is now applied to any sealed-unit disk drive with aerodynamic head design.The
Winchester disk is commonly found built in to personal computers and workstations, where it is referred
to as a hard disk.