Since the obvious objective in using a hard disk is to transfer data to the hard drive
and onto the disks, or off the disks and out of the drive, the rate of data transfer is of
paramount importance. Traditionally, real transfer rate metrics have been very underrated
and given almost no attention compared to positioning specifications like seek time. The only transfer specification that is really commonly
mentioned is the speed of the interface, which is actually the least important
indicator of overall disk performance.
Before we look at transfer specifications, we need to have a short word about
terminology. 
Transfer rates are confusing in part because of the phrase "transfer
rate" can mean so many different things. Data transfer occurs in two main steps. For
a read, data is first read from the disk platters by the heads and transferred to the
drive's internal buffer; then it is moved from the buffer, over the interface, to the rest
of the system. For a write, the process is reversed. The rate that transfer occurs within
the disk is of course the internal transfer rate; the rate that transfer occurs
over the interface is the external transfer rate. They are usually not the same,
and in some cases can differ by an order of magnitude.
Internal transfer rates are further broken down into the media transfer rate and the
sustained transfer rate, and further complicating things is the fact that transfer rates
are not constant over the surface of the drive. It sounds impossible to get a handle on,
but it's not that bad once you place it all in the proper context and perspective, and
that's exactly what we will do in this section.
Tip: Whenever you are
reading a spec sheet, or discussing transfer rates with someone, be sure to find out
exactly what transfer rate is being discussed. By itself the term "transfer
rate" is meaningless.
Next: Internal Media Transfer Rate
