Hardware Reference
In-Depth Information
How Magnetic Disks Store Data
Magnetic storage, such as that used with a traditional hard disk, creates transitions by
magnetizing areas of the disk with either a positive or a negative polarity. It then reads the
transitions between the positive and negative areas as 1s and the lack of transitions as 0s.
The read/write head, although it's a single unit, actually performs two different
functions: it magnetizes areas of the disk, and it reads changes in polarity and relays them
to the drive controller.
The disk surface is coated with a thin layer of iron oxide particles. The read/write
head has a wire coil around it, and electricity passes through that coil. The magnetic
fi eld polarizes the surface of the disk as either positive or negative. The fl ow of electricity
through the wire then reverses, changing the polarization and creating a transition point.
Internal vs. External Hard Disk Drives
External hard drives connect to a PC either via a USB or FireWire (IEEE 1394) connector.
They're easy to use; you just plug them in. External hard drives often require their own
power source from an AC adapter.
It's possible to set up a system to boot from an external hard disk, so you can put the
OS on an external hard disk. However, you'll lose performance doing so, because an external
hard disk is usually slower than an internal one due to the bottleneck of the external cable
and connector. In addition, it makes the computer unusable without the external drive, so if
the external drive is missing or misplaced, the computer won't work. External hard disks are
also more susceptible to theft than internal ones.
How much slower is an external drive than an internal one? Universal
Serial Bus (USB) version 2.0 has a data rate of up to 480 megabits
per second (Mbps). FireWire comes in at 400 Mbps. In contrast, Serial
Advanced Technology Attachment (SATA), used for internal drives, can
transfer data at between 1.5 and 6 Gbps.
Factors that Govern Hard Disk Speed
Speed of hard disk access is an important consideration in system performance, because a
hard disk, being a mechanical device, is likely to be a bottleneck area. The processor and
RAM can run much faster than the fastest hard disk, so they end up waiting around for the
hard disk to supply the needed data.
As mentioned in the previous section, internal hard disks are much faster than external
ones. Some other factors that affect hard disk speed are as follows:
Revolutions per Minute (RPM)
The faster the platters spin, the faster the read/write heads
can read from them. RPMs on hard disks range from 5,400 to 15,000. Obviously, this isn't
an issue on solid-state drives, which use static RAM to store the data rather than discs
because there are no platters.
