• How do hard drives work?
• What is a solid-state drive?
• Capacity
• Interface
• PATA
• SATA2
• USB 2.0
• FireWire
• eSATA
• Network-attached storage
• Form factors
• Internal or external?
• Hard drive specifications
• RPM
• Disk cache
• Native command queuing (NCQ)
• Seek/access time
• MTBF
• Warranty
• Noise
• Buying the right hard drive
• File systems, fragmentation and slow downs
• RAID

<---> Noise

Noise ratings are rarely quoted (unless for marketing purposes), but are an important consideration. Internal hard drives can sometimes be the loudest component of a computer system.

Most hard drive vendors rate hard drive noise in decibels. A very quiet drive will have a peak sound output of less than 25dB. This is about the point at which humans can discern sound, making a drive with less than 25dB effectively silent. Solid-state drives are completely silent.

<---> Buying the right hard drive

The performance of a hard disk is determined by many factors. The interface and spin speed are the best indicators that we, as consumers, have of the speed of a hard disk. It's often hard to tell how fast a drive is without proper benchmarking, so we recommend researching and reading reviews first. Buying the correct drive often comes down to whether you value speed or capacity more.

<--->File systems, fragmentation and slow downs

A hard disk merely provides a way of storing data. Drives don't tell the PC how to organise it; that's the job of the file system.

A file system is essentially a directory of the data stored on a hard disk. It is the PC's way of remembering where files are physically located on the disk — for instance, that the file "fluffythecat.jpg" is located on track 31, sector 18 of the hard disk.

File systems perform differently and have other differences, such as the maximum capacity of disk they can support, support for encryption and how the geometry of the hard disk is divided up into addressable regions. The superseded FAT32 file system, for example, only supports a maximum capacity of 32GB, and no single file can be bigger than 4GB.

One of the most important aspects of a file system is how it deals with fragmentation. Fragmentation occurs when a file tries to fit into a gap on the hard disk that is too small for it. Say you delete a 10Kb file, which leaves a gap free on the hard disk. Then you try and write a 15Kb file. Only the first 10Kb can fit where the old file went. The rest has to go elsewhere on the disk, thus fragmenting the file.

Fragmentation will require that the head jump from one part of the disk to another in order to complete the file reading. For this reason, defragmenting your drive periodically is important. Defragging will attempt to realign files so that the entire file can be found in one place. Windows offers an integrated defragmentation tool, though third-party software can occasionally provide better hard drive optimisation. Mac users don't have to defragment their drives because they are not as prone to fragmentation.

One other thing that users often find is that hard disks get slower as they fill up. There is a very good reason for this: shorter tracks. Hard disks write first to the outside tracks of the platter, and then work their way in towards the centre. The outside tracks have a larger circumference than the inner tracks, and therefore each revolution of the hard disk covers more area on the outer tracks than it does on the inner (and thus passes more data to the read head). As the disk fills up, more data is being written to the slower inner tracks, and so you'll see an overall decrease in speed on that data. There's no real solution to this problem, except to keep your PC as free from clutter and excess data as possible.

Defragmenting the drive will move all the data to the outer tracks. Otherwise, the general rule of thumb is not to fill the hard drive to within 15 per cent of its formatted capacity. The file system is often the biggest thing standing in the way of a hard drive being compatible between operating systems. Though the FAT32 file system is supported by Windows and MacOS X, both operating systems use different file systems for their system drives. Windows has used the NTFS file system since Windows 2000, while MacOS X uses the HFS+ Journaled file system.

Mac-formatted external hard drives (usually those that have FireWire 400 and 800 connections) won't work on Windows without reformatting to the NTFS file system. Mediafour's MacDrive program will allow users to access HFS+ Journaled drives from Windows.

RAID

There are several methods of combining multiple hard drives into a single volume. RAID — standing for Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks — lets users boost drive speed or help to prevent data loss in the event of one or more drive failures. Some basic RAID features are supported by motherboards and multiple-drive external hard drives or NAS devices, but you may need to purchase a hardware RAID card.

RAID is entirely a function of the disk controller; a hard disk does not need to actively "support" RAID in order to work in an array. There are several different levels of RAID, each serving different needs.

RAID 0, called striping, is where part of a file is stored on each disk. When the data is read, it is read in parallel from the disks in the RAID. Because each disk has to do less work, the overall speed of the file transfer is greater. RAID 0 can give you significant performance improvements. The capacity of a RAID 0 array is equal to the equivalent of the number of drives in the array multiplied by the capacity of the smallest drive. In other words, each drive has the same formatted capacity. The drawback to RAID 0 is that if one drive dies then all the data in the array is lost, so it is rarely used for vital information.

RAID 1, or mirroring, replicates all the data across all the drives in the array. This gives you reliability and speed (since you can still read data in parallel off multiple drives), but reduces your capacity. Two 200GB drives in a RAID 1 array, for example, would offer a total capacity of only 200GB, because all of the data is replicated. If one of the drives dies, then no data is lost (because all the data is still on the other). RAID 1 provides speed and reliability but comes at a significant cost in capacity.

To get the best of both RAID 0 and RAID 1, a third RAID level, RAID 1+0, is used. The array, which requires four drives or more, involves mirroring the data written in pairs, so that two of the drives in the array are striped in RAID 0 format, while also having a mirror of the data similar to RAID 1. Since this requires a comparatively large number of drives, it is relatively uncommon in desktop PCs.

The other common RAID formats — RAID levels 5 and 6 — are extensions of RAID levels 0 and 1 across arrays of varying sizes. These are available on 4-8 bay NAS devices.

One other method of combining multiple hard drives into a single volume is through a process dubbed concatenation. This method, referred to as a JBOD (Just a Bunch of Disks) array, simply tells the operating system that the multiple drives are one volume, even though they aren't. This method does not provide any striping or mirroring; data is simply to the drives in sequential order. It will fill the first drive before moving onto the next.