All modern hard disks have an internal buffer, or cache, that is used as an
intermediate repository for data being transferred between the hard disk and the PC. It is
described in detail in this operation section. The
size of this buffer is usually given as a standard specification on modern drives.
Having some cache in a drive is somewhat important to overall performance; the
drive will use it to buffer recent requests and to "pre-fetch" data likely to be
requested by the system in the future. If this data in the cache is in fact needed, it
will be transferred to the system at the drive's external transfer rate--much faster than
would be possible if there were no cache. However, the number of requests that fit into
this category is relatively small. Increasing the size of the cache even by a substantial
percentage doesn't change this very much, because no matter how large the cache, it will
always be a very small percentage of the total capacity of the drive. Caches today,
despite significant increases in size, are still far less than 0.1% of the size of the
disk drives they serve.
As memory prices have fallen into the "dirt cheap" range, drive manufacturers
have realized that they can increase the size of their buffers at very little cost.
Certainly nothing is lost in doing this; extra cache won't hurt performance; but
neither does it greatly improve it. As a result, if interface
transfer rate is the "reigning champion" of overrated performance
specifications, then cache size is probably the "prime contender". 
Some
people seem to think a 2 MiB buffer makes a drive four times as fast as one with a 512 kiB buffer! In fact, you'd be hard pressed to find even a 4% difference between them in most cases, all else being equal. Not surprisingly, both external transfer rate and cache size are overrated for the same reason: they apply to only a small percentage of transfers.
The cache size specification is of course a function of the drive's cache characteristics. Unfortunately, manufacturers rarely
talk about any characteristics other than the cache's size.
Next: Drive Start-Up Ready Time
