Hard disks can have one platter, or more, depending on the design. Standard consumer
hard disks, the type probably in your PC right now, usually have between one and five
platters in them. Some high-end drives--usually used in servers--have as many as a dozen
platters. Some very old drives had even more. In every drive, all the platters are
physically connected together on a common central spindle, to form a single assembly that
spins as one unit, driven by the spindle motor. The platters are
kept apart using spacer rings that fit over the spindle. The entire assembly is secured
from the top using a cap or cover and several screws. (See the spindle
motor page for an illustration of these components.)
Each platter has two surfaces that are capable of holding data; each surface
has a read/write head. Normally both surfaces of each
platter are used, but that is not always the case. Some older drives that use dedicated servo positioning reserve one surface for holding servo
information. Newer drives don't need to spend a surface on servo information, but
sometimes leave a surface unused for marketing reasons--to create a drive of a
particular capacity in a family of drives. With modern drives packing huge amounts of data
on a single platter, using only one surface of a platter allows for increased
"granularity". For example, IBM's Deskstar 40GV family sports an impressive 20
GB per platter data capacity. Since IBM wanted to make a 30 version of this drive, they
used three surfaces (on two platters) for that drive. Here's a good illustration of how
Western Digital created five different capacities using three platters in their Caviar
line of hard disk drives:
Model
Number |
Nominal
Size (GB) |
Data
Sectors Per Drive |
Platters |
Surfaces |
WD64AA |
6.4 |
12,594,960 |
1 |
2 |
WD102AA |
10.2 |
20,044,080 |
2 |
3 |
WD136AA |
13.6 |
26,564,832 |
2 |
4 |
WD172AA |
17.2 |
33,687,360 |
3 |
5 |
WD205AA |
20.5 |
40,079,088 |
3 |
6 |
Note: In theory, using only
one surface means manufacturing costs can be saved by making use of platters that have
unacceptable defects on one surface, but I don't know if this optimizing is done in
practice...
From an engineering standpoint there are several factors that are related to the number
of platters used in the disk. Drives with many platters are more difficult to engineer due
to the increased mass of the spindle unit, the need to perfectly align all the drives, and
the greater difficulty in keeping noise and vibration under control. More platters also
means more mass, and therefore slower response to commands to start or stop the drive;
this can be compensated for with a stronger spindle motor, but that
leads to other tradeoffs. In fact, the trend recently has been towards drives with fewer
head arms and platters, not more. Areal density continues to increase, allowing the
creation of large drives without using a lot of platters. This enables manufacturers to
reduce platter count to improve seek time without creating drives too small for the
marketplace. See here for more on this trend.
|
This Barracuda hard disk has 10 platters.
(I find the choice of fish hooks as a
background for this shot highly amusing.  ) |
Original image © Seagate Technology
Image used with permission. |
The form factor of the hard disk also has a great influence on
the number of platters in a drive. Even if hard disk engineers wanted to put lots of
platters in a particular model, the standard PC "slimline" hard disk form factor
is limited to 1 inch in height, which limits the number of platters that can be put in a
single unit. Larger 1.6-inch "half height" drives are often found in servers and
usually have many more platters than desktop PC drives. Of course, engineers are
constantly working to reduce the amount of clearance required between platters, so they
can increase the number of platters in drives of a given height.
Next: Platter Substrate Materials
