The spindle motor is one of the most important components in the hard disk, because its
quality and power have a direct impact on many key performance and reliability concerns. It is discussed in detail in this section.
The drive's spindle speed affects both positioning and transfer performance and is thus one
of the most important directly-quoted performance specifications unto itself; it is
described in its own specification section. It
affects positioning performance because it directly correlates to the latency of the drive (and in fact, is the only factor
that affects the latency of regular hard disk drives). Latency, in turn, is an important
component of access time, the specification that best
correlates to overall positioning performance. Spindle speed affects transfer position
because it is related to the drive's media transfer rate,
which is the prime transfer performance specification.
Note: While spindle
speed affects the media transfer rate, it is not proportional to it. The reason
is that it is more difficult to read and write at very high linear densities when running at very high
spindle speeds. This means that in some cases a drive running at 5400 RPM will have a
higher areal density than a similar drive running at 7200 RPM. The transfer rate is still
normally higher for the 7200 RPM drive, because the spindle speed is 33% higher and the
linear areal density is usually only smaller by a factor of 10% or less (though this could
change at any time; who knows what those engineers are up to! 
)
The power of the spindle motor has an impact on the drive's spin-up speed, for obvious reasons. Also, since the
spindle motor is the primary consumer of power in the hard disk, its design has the
biggest impact on overall power consumption.
In some ways, slower drives have an advantage here; it takes longer to spin anything up to
10,000 RPM than to 5400 RPM (unless you use a correspondingly larger motor in the faster
drive.)
Next: Data
Recording and Encoding Factors
