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Hard Disk Drives
Description: 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.
Tip: For additional information
on hard disk drives, including much more discussion of many of the technical details,
criteria and features mentioned below (and other related topics), see the extensive Reference Guide section on hard disk
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 should!
Related Components: 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
- 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 issues.
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 compatible.
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 engineering estimates.
- 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
- 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
- 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
Important Features: 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 system performance.
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 warranty issues.
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 failure.
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.
Warranty Issues: 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 dealers.
- 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
- 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 capacity.
- There can be complications when upgrading certain types of PCs, especially ones that use
proprietary components. Some require specific makes or models of drives.
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