The hard disk drive is the "data center" of your PC--it holds
all of your programs and data. The CPU may be the "brain" of
your system, but if so, the hard drive is its memory and personality--it
is what makes your PC what it is. Few of us could imagine using a PC
without a hard disk drive; it's a defining component of the modern PC.
Role and Subsystems: The hard disk
is a key component in the storage subsystem. It plays an important role in
the performance, capacity and software support of the PC. Its quality and
reliability is probably more important than that of any other component,
since it is the only component that you cannot just swap out with another
of the same type in the event of failure--you lose your data in the
process. It therefore commands significant attention, or at least, it
The hard disk must be matched to several other components in the system.
First, it must be of a form factor compatible with the system case.
Second, it must use the same interface as the controller or controllers
within the PC. Finally, the selection of hard disk interface will have an
impact on the selection of other storage subsystem components such as the
CD/DVD drive. Usually the interface is chosen for the system and the hard
disk selected to match it. Most systems use IDE/ATA, for which controllers
are provided on the motherboard and no other components are necessary for
compatibility. System case compatibility is also fairly universal.
Key Compatibility Selection Criteria:
There are several important criteria to keep in mind when selecting a hard
disk to suit your system, but most systems use standard hardware, making
it actually fairly easy to find hardware that will suit your needs:
- Form Factor: Hard disks come in
several different common sizes, matched to the bays in the system case
where they are installed. For most desktop PCs, the standard is the
3.5" wide form factor (actually about 4" wide, but that's
the name). Standard hard disks are called slimline drives and
are 1" in height. For notebooks the standard is the 2.5"
form factor. Most hard drives sold for PCs, and almost all for regular
consumer PCs, are 3.5" slimline units.
- Interface Type: Most hard drives
use the IDE/ATA interface; others, especially in high-end systems, use
the SCSI interface. Those two cover 99% of the drives in desktop PCs,
though there are some external drives that interface through the
parallel, USB, or PC Card ports (they are used more for notebooks than
desktops, actually). For most systems, you want an IDE/ATA hard disk.
If you decide to buy or build a SCSI system, you need to purchase a
SCSI host adapter, and make sure that you get matching drives that use
a "flavor" of SCSI that the host adapter supports.
- Spindle Speed Support and Cooling:
Some high-speed drives generate a significant amount of heat and may
require special cooling. Some system cases, especially smaller ones,
may not be able to use these faster drives without problems.
Fortunately most newer drives have largely addressed these heat
Performance and Capacity Selection
Criteria: There are many different
performance issues related to hard disks. I discuss them in gory detail
here, but for selecting a new hard drive I can boil the capacity and
performance issues down to the following key points:
- Capacity: Obviously, the size of
the drive is an important consideration: bigger drives hold more. At
the same time, don't overspend on an enormous drive unless you do a
lot of downloading or working with large graphics, audio and video
files. For "typical" PC uses a 50 GB drive will remain
mostly empty for most people. (Of course, every time I think
"drives are now big enough", a few years later I find that
I'm wrong as I find new ways to fill them up!)
- Spindle Speed: One of the most
important differentiators between different drive models from a
performance standpoint is the speed at which the spindle spins (and
thus the data platters as well). For IDE/ATA, the fastest drives spin
at 7,200 RPM; slower ones go at 5,400 RPM or even 4,400 RPM. Faster
drives provide both higher transfer rates and faster random access to
your data. For SCSI, faster 10,000 RPM and even 15,000 RPM drives are
available (at a significantly higher cost, of course).
- Seek Time: This specification,
given in milliseconds, refers to how quickly the hard disk's actuator
(the device that moves the head assembly) can position the heads to a
random place on the surface of the disk platters. It is a very
important performance specification; the lower the number the better
and even differences of 1 millisecond can make a difference in
performance in some situations.
- Areal Density: This specification
refers to how much data the drive packs onto each of its platters. It
can be found listed in the product manual for the drive, or you can
sometimes estimate it for comparing drives of the same form factor by
dividing the size of the drive by the number of platters inside the
drive. For example, the largest of the Maxtor DiamondMax 80 series of
drives packs 80 GB onto four platters, giving it a rough-cut density
of 20 GB per platter. Newer drives pack data more densely, improving
both capacity and performance. (Note that some drive sizes in a family
may use half of one platter or even some other fraction, so be careful
in doing these calculations).
- Sustained Transfer Rate: This
specification indicates how fast the drive can stream data off the
surface of the platters during sustained reads of many megabytes of
data in a row. The higher this figure the better, but small
differences between models usually aren't significant. It is most
important for those working with large files.
- Interface Speed: Both IDE/ATA and
SCSI operate at several standard speeds. For IDE/ATA drives the
current standard is Ultra ATA/100, with the "100" standing
for the interface's maximum potential throughput, 100 MB/s. In theory,
faster interface drives are better, but for most PCs the speed of the
interface has little impact on overall hard disk performance, as long
as the number is above the maximum sustained transfer rate of the
drive (see under "magic numbers" below for more). If you
want to run an Ultra ATA/100 drive you need a controller that supports
Ultra ATA/100, but even a controller that runs at 66 or 33 MB/s
theoretical bandwidth will work OK; the drives are backward
Quality Selection Criteria:
The hard disk is the only component in the PC where a failure means you
have a much bigger problem on your hands than merely getting replacement
hardware--you have to worry about your data! For this reason, quality is
probably more important for this component than it is for any other. Also
note that company reputation is a very important quality consideration.
Here are some specific quality criteria to look at when evaluating models:
- MTBF: As with power supplies and
some other components, MTBF for hard disks stands for mean time
between failures. It gives an estimate of the quality of the drive
by approximating the number of hours that will elapse between failures
when a group of drives of this type are run for millions of aggregate
hours under ideal conditions. It does not mean how long you should
expect any particular drive to continue running. A model with a
significantly larger MTBF figure can be reasonably predicted to last
longer than one with a smaller value, but remember that these are just
- Service Life: The manufacturer's
designed life expectancy of the drive. For the stated number of years,
the manufacturer of the drive believes the unit will work reliably and
safely; beyond that point the drive may continue to work but the
chance of problems increases significantly.
- Warranty Length: The number of
years the manufacturer warrants the drive. Watch for discrepancies
between this figure and the service life of the drive--whichever is
lower is what the prudent person will trust!
- Warranty Policies: Some companies
provide much better warranty service and coverage than others, and I
consider this a quality indicator. See the "Warranty Issues"
section below for more.
- Noise and Vibration: As spindle
speeds and actuators get faster, drives make more and more noise
unless careful engineering is done to counteract it. For some people
this is not much of an issue, but for others it is very important.
Drives come with objective noise specifications, and most reviewers
also assess the amount of noise made by the units they are evaluating.
If you are especially sensitive to noise, check out in person the
sound level of any model you are considering before you buy; it seems
to be a very personal issue.
- Quality and Reliability Features:
Most drives come with a number of features that are designed to
improve the integrity and reliability of drives. A few of the more
important ones you may wish to look for are: "SMART"
technology, enhanced shock protection, head load/unload technology,
temperature monitoring, and enhanced automatic defect mapping.
Hard disks perform a very specific function within the PC, and there
aren't typically features put on them to differentiate models as is the
case with many other components. The most important "features"
of a hard disk for most people are performance, capacity and reliability,
and specific features added to drives are usually oriented around
improving quality and reliability as described just above.
"Magic Numbers" To Watch For:
Hard disks have more than their fair share of magic numbers, and
fortunately, some of them are actually important. ^) Here are the ones I
see most frequently:
- Capacity: Obviously this is
important, as described above, though it is often presented with no
performance specifications, as if capacity was all that mattered.
- Spindle Speed: This is also
reasonable as magic numbers go. It certainly doesn't tell the whole
story, but if you were going to provide only one performance
specification, that would be the one for my money.
- Interface Transfer Rate: This is
the most overrated performance specification for hard disks, and the
one most often used as a selling point. Today's IDE/ATA drives (and
controllers) are usually sold as "Ultra ATA/66" or
"Ultra ATA/100", where the "66" and
"100" are interface transfer rate specifications in MB/s.
Unfortunately, in most cases the real performance of the PC is based
primarily on the internal performance characteristics of the drive
itself, not the interface. The most important thing is to keep the
interface transfer rate above the maximum sustained transfer rate of
the drive; the rest is gravy, and there are no drives on the market in
2000 that can saturate a 66 MB/s interface, much less a 100 MB/s
interface. Don't worry too much about the interface transfer rate on a
modern PC, unless you are using a high-end SCSI system with multiple
fast drives on it.
- Buffer Size: This refers to the
size of the memory buffer or cache within the hard disk. Larger
buffers improve performance by a small degree, but not nearly to the
extent that some manufacturers would have you believe. A 4 MiB buffer
may be four times the size of a 1 MiB buffer but it will likely only
make a difference of a couple of percentage points in the performance
of the hard disk subsystem. Don't be fooled.
Performance Impact: The
hard disk is a very important performance component within the PC. The
reason it impacts performance so substantially is that it is a mechanical
component, and is therefore thousands of times slower than other key
performance components such as the CPU and system memory, which are purely
electronic. Upgrading to a newer hard disk can noticeably improve overall
Retail, OEM and Gray Market Issues:
Hard disks are commonly sold in either retail or OEM packaging, and there
are important differences between them. For starters, retail drive
packages usually come with an installation kit, including a cable,
mounting screws, jumpers, and a driver disk; OEM drives usually just come
as a bare drive. You may need the retail package in order to easily
install some drives, though the hardware is inexpensive and the drivers
available from the manufacturer's web site in most cases.
The bigger issue is related to the
warranty. Some manufacturers will provide no warranty support on OEM
drives (which is why they are cheaper). Others provide a three-year or
five-year warranty on all their drives, OEM or retail (but for these OEM
drives are often not cheaper--TANSTAAFL). Most manufacturers will also
refuse to provide warranty coverage on gray market drives. Be sure to shop
around, as in many cases retail-packaged drives can be found for only a
few dollars more than OEM drives.
Importance of Manufacturer:
There are always some people who insist that one manufacturer makes
higher-quality drives than another based on personal experience--and
someone else who says the exact opposite. The reality is that in most
ways, quality units are made by any of the half-dozen or so big-name hard
disk manufacturers. The biggest differences between manufacturers are
related to warranty coverage and warranty policies. See below for more on
Typical Component Lifetime:
The reliable service life of a typical consumer-level hard disk drive is
around three to five years. Some drives work for a decade or longer, but
every year that passes after three or so increases the chances of a
Hard drives go obsolete very slowly: you
can plug a 10-year-old IDE/ATA drive into a modern system and make it
work. However, bigger and faster drives come out every year, so if you
care a great deal about performance you will probably find yourself
wanting to upgrade to a newer drive in a few years.
Driver Support Issues:
To enable the faster transfer modes drivers may be required for some
operating systems, especially older ones. These are fairly standard and
aren't a big issue any more (though they were at one point). Note that
this matter doesn't affect the actual operation of the drive, just its
interface to the rest of the PC, and with newer operating systems there
isn't really an issue anyway in most cases.
There are two important warranty issues related to hard disks (see here
for much more on this topic):
- Warranty Coverage: Unpleasant
surprises related to warranty status are probably more common with
hard disks than with any other component; this is largely due to the
matter of OEM drives, which are flooding the marketplace. If you want
to sure of warranty coverage on your drive, be careful to purchase
either a drive from a manufacturer that has a "no questions
asked" warranty policy, or buy a drive that specifically includes
consumer warranty coverage from a dealer authorized by that
manufacturer. Be wary of very cheap drives from less reputable
- Warranty Policies: Some
manufacturers have much better policies than others when it comes to
warranty support. If this is important to you, the first issue is to
investigate if the manufacturer you are contemplating traditionally
replaces failed drives with new ones or refurbished units, and if the
latter, what sort of success people typically have had with them (in
some cases these have a lot of problems). The second issue is how
quickly the company will replace a failed drive within the warranty
period: some are good about immediately cross-shipping replacements,
while others will make you wait at a time you can least afford it.
Special Specification Considerations:
Here are some additional tips to keep in mind:
- If you are going with a SCSI hard disk
you need a SCSI host adapter, and you may want to go with a SCSI
optical drive as well.
- Some older systems have BIOS or
operating system capacity limits that can cause problems if upgrading
to new, large hard disks, especially those over 8 GB in size.
- Hard disks improve in performance
rapidly. Don't buy an older model drive if you can get a newer one
that is faster for only a few dollars more.
- Don't buy used hard disks. There's
absolutely no way to tell if there is a problem with the drive by
looking at it, and even running it for a day or two may not reveal
problems that the prior owner has decided not to tell you about. It
may be fine, but it may not--and it's your data on the line.
- New drives aren't always faster than old
ones--there is a trend now towards large, slower drives for
low-performance "appliance" applications, so check those
- You may need mounting rails to put a
3.5" drive into a 5.25" drive bay on certain system cases.
Most computer stores sell these for under $10.
- Larger drives cost more than smaller
ones of the same drive family, but generally cost less per GB of
- There can be complications when
upgrading certain types of PCs, especially ones that use proprietary
components. Some require specific makes or models of drives.