Whenever a hard disk is transferring data over the interface to the rest of the system,
it uses some of the system's resources. One of the more critical of these resources is how
much CPU time is required for the transfer. This
is called the CPU utilization of the transfer. CPU utilization is important
because the higher the percentage of the CPU used by the data transfer, the less power the
CPU can devote to other tasks. When multitasking, too high a CPU utilization can cause
slowdowns in other tasks when doing large data transfers. Of course, if you are only
doing a large file copy or similar disk access, then CPU utilization is less important.
CPU utilization is usually highest when running an IDE/ATA hard disk using a programmed I/O mode, and lower when using a DMA mode. Most newer systems use DMA modes for
hard disk data transfer, provided that the appropriate hardware and drivers are installed.
SCSI drives use a dedicated controller (host adapter) and typically also have low CPU
utilization.
CPU utilization is one of those performance factors that is both grossly underrated and
overrated at the same time. 
Most people have never even heard of it; it often seems
though that a big percentage of those who do understand its role worry about it way
too much. Like most performance issues, sweating small differences in numbers is
usually pointless; it doesn't matter much if your CPU utilization is 5% or 10%; but if it
is 80% or 90% then you are going to see an impact on the usability of the system if you
multitask.
Another key issue is that faster drives transfer more data, and more data--all else
being equal--requires more processing time. It's totally invalid to compare the CPU
utilization of drives of different generations without correcting for this very important
consideration.
One hard disk utility commonly employed for testing CPU utilization is HD Tach. (Note that this should be considered as information, not an endorsement!)
Next: Command Overhead and Multiple Device Considerations
