All interfaces are limited in terms of the number of devices they can support, but some
are more limited than others. This is rarely perceived to be an important issue by most
new PC users, but after a couple of years of accumulating devices it can rather suddenly
grab your attention. 
For example, the standard IDE/ATA controllers included in most
systems will support a total of four devices. Most PCs come with a single hard disk and
one optical drive, leaving room for two more devices. No problem right? Not at first.
However, you may decide to add to this a Zip drive and then perhaps a CD-RW unit. Then,
two years down the road, you might realize you want to add a new hard disk... This happens
to a lot of people who never expected they would ever need more than four IDE/ATA devices.
Expanding a system to allow for more IDE/ATA devices can be done; in many cases it is
fairly simple to do, but in others it is far more involved. In contrast, some interfaces
are relatively easy to expand. For example, once one has a SCSI host adapter and a SCSI
bus running, adding a new device to the existing SCSI bus is fairly trivial, and support
is provided for 7 or 15 different devices in a single chain.
Easy expansion is also part of the appeal of specialty interfaces
like USB: you simply plug the device in and load a driver, and
off you go! This makes it very flexible for a variety of uses. For systems like notebooks,
USB, PCMCIA or the parallel interface may be selected for additional hard disks or similar
devices primarily because of the limited expansion options of the IDE/ATA implementations
on such systems.
Next: Cost
