The internal media transfer rate of a drive (often just called the media
transfer rate or the media rate) refers to the actual speed that the drive
can read bits from the surface of the platter, or write bits to the surface of the
platter. It is normally quoted in units of megabits per second, abbreviated Mbit/sec or
Mb/s. Typical values for today's drives are in the hundreds of Mb/s, with a maximum media
rate of about 500 Mb/s being high-end at the time of this writing.
Note: Media transfer
rates are not normally specified in megabytes per second, even though sustained transfer rates are; this is not
100% consistent, however. Watch out for this discrepancy when looking at the numbers.
Media transfer rate can be confusing to understand even for the serious hard disk
enthusiast; it's equally difficult to describe. 
For starters, let's explain what it is
not. It is only related to what is going on inside the hard disk, and therefore
has nothing directly to do with the interface
transfer rate. It differs from the sustained
transfer rate in that it refers only to the speed of reading or writing bits
to a single track of one surface of the disk. Nothing else is included--no
positioning, no track or head switching. A track holds a relatively small amount of
data--under 0.25 MB with current technology. This means that almost no real-world reads or
writes occur on a single track except for very short files, and the performance when
reading those is primarily limited by positioning, not transfer. The end result of this is
that the media transfer rate does not have much relevance to real-world use of a drive. It
is primarily a "theoretical" specification that illustrates the state of the
drive's technology. It is used almost exclusively for comparing drives against each other.
It is also the basis for the calculation of the sustained
transfer rate specification.
Media transfer rates are not constant across the entire surface of a platter. Let's
recall for a moment the fact that modern disk drives use zoned bit recording. This is done because the
length of the inner tracks on the disk is much shorter than that of the outer tracks. ZBR
allows the outer tracks to have more sectors per track than the inner tracks. However,
since every track is spinning at the same speed, this means that when reading the outer
tracks, the disk is transferring more data per second when when reading the inner tracks.
For this reason, the media transfer rate decreases as you move from the outer tracks of
the disk to the inner ones.
The explanation above is the reason that there is no single "media transfer
rate" figure for a modern hard disk. They are typically stated as a range, from
minimum to maximum (with the maximum figure given alone, of course, if only one number is
provided). For example, the IBM Deskstar 34GXP (model DPTA-373420) has a media transfer
rate of between approximately 171 Mb/s and 284 Mb/s depending where on the disk you are
reading: that drive has 12 different zones. This drive has 272 sectors in its innermost
zone, and 452 sectors on its outside tracks.
Another important thing to remember about the media transfer rate (and another reason
why it is a theoretical measure only) is that it includes all bits read or
written to the disk, not just user data. As
discussed in detail here, some of the data storage space in a sector is reserved for
overhead. This means that you cannot assume that the media rate represents the rate at
which user data can be read from the disk. Taking the IBM drive above again as an example,
its maximum media transfer rate is 284 Mb/s, but the maximum rate that the drive can read
user data is about 222 Mb/s in the outside zone.
It's not really feasible to calculate the media transfer rate from other drive
specifications, because manufacturers typically do not publish details of their sector
format and other pertinent overhead characteristics. The best that you can do is
approximate the value by looking at the rate at which user data can be streamed from a
given part of the disk. To so do so, we need to know how much data is able to pass under
the read/write heads in one second. This is dependent on the density of the data (how
tightly packed the data is into each linear inch of disk track), and also how fast the
disk is spinning. The density of the data can be calculated easily if we know how many
sectors are on the track, since we know how many bytes of user data there are in a sector
(512). The speed of the disk is calculated in RPM, so we divide it by 60 to get
revolutions per second. This gives us a calculation of the data transfer rate in megabits
per second as follows (to get the result in megabytes per second, simply divide by
8):
User Data Transfer Rate = (Spindle Speed / 60 * Sectors Per Track * 512 * 8) /
1,000,000
This formula shows the derivation of the 222 Mb/s figure above: use 7200 for the
34GXP's spindle speed, and 452 sectors on its outside tracks. Note that you need the true
physical geometry here; the logical BIOS setup
parameters will give incorrect results. (If the geometry you are using says the disk has
63 sectors per track and 16 heads, it's almost certain that you are looking at the logical
BIOS geometry!) And again, remember that this is not the same as the media transfer rate;
to get that figure you'd have to replace the "512" above with the total number
of bits, including overhead, contained in each sectors of the disk.
The media transfer rate of the drive is primarily affected by all of the various data recording and encoding factors, as well as the size of the platters, and the drive's spindle speed. In addition, the drive's controller must be fast enough to be able to handle
the fastest rate that the disk can read or write, but manufacturers ensure that this is
never an issue by beefing up their controllers where necessary.
Next: Head Switch Time
