The amount of time that is required for the disk platters to get up to full operational
speed from a stationary start is called the drive's spin-up time. It is usually
specified in seconds.
The importance of this specification depends entirely on how you use your hard disk. If
you are the type of user that starts up the computer and then leaves the disks spinning
all the time, this figure holds virtually no significance. On the other hand, if you use power management to spin down the disk after a couple
of minutes of idle time, then whenever you go to use the disk again after an idle period,
you will have to wait for the drive to spin back up to speed again. This spec is more
important for notebook PC users than those who use desktops, since power management is
more important for notebooks, and is more commonly used. To be honest though, either way,
a drive with slow spin-up time really represents more of an inconvenience than a major
performance issue. After all, once the drive is actually running in normal use the spin-up
time becomes irrelevant.
Spin-up time is of course a component of start-up
ready time, since in order to get ready when the drive starts, the platters must spin
up. 
It is a function primarily of the drive's spindle
characteristics and the number and size of the
platters. Smaller drives tend to have lower spin-up times, in part due to lower mass
and in part because they are used more for mobile applications.
Next: Power Consumption
